Pharmacists recently conducted a study with respect to the reasons the

Official Explanation

This argument does not consider that an outside factor may cause some people to have both poor vision and dry eyes. Selection (A) provides an outside factor
for both conditions. It is the correct answer. Selection (D) is tempting, but don’t be misled.

Even if most people who wear contact lenses do not have dry eyes, this does not weaken the argument because it is based on the incidence of dry eyes between people who wear contact lenses and those who do not. Secondly, the author does not argue that wearing contact lenses must cause dry eyes, only that it can cause dry eyes.

Hi Bunuel.
Aren't C and D against the premises, warranting that both are wrong answers?

Hello expert,
The OE of this question is not complete, just addressing D, so need your opinion to B. I think both A and B can weaken the argument with alternative cause, so why B is inferior than A? Thanks in advance.

Here's the support for the conclusion:

Dry eyes were more frequently experienced by customers who wore contact lenses than by customers who did not wear contact lenses.

So, there is a DIFFERENCE between the frequency of dry eyes in customers who wear contact lenses and and the frequency in customers who do not wear contact lenses, and the conclusion is based on that difference.

Now, here's (B).

(B) Physical exertion causes dry eyes in many people who wear contact lenses.

Notice that (B) doesn't really change what we know. Regardless of what (B) says, it is still the case that dry eyes were more frequently experienced by customers who wore contact lenses than by customers who did not wear contact lenses, and (B) does not provide AN ALTERNATIVE EXPLANATION FOR THE DIFFERENCE between the frequencies of dry eyes. After all, we have no information indicating that customers who did not wear contact lenses do not ALSO experience dry eyes because of physical exertion. In other words, what (B) says could be THE SAME for people who do not wear contact lenses.

Now, let's consider (A).

(A) An inherited condition can cause both weak eyesight and dry eyes.

Notice that (A) does provide an alternative explanation for the difference between the frequencies of dry eyes. It could be that the inherited condition causes both the wearing of contact lenses and dry eyes of people who wear contact lenses, making these people different in both ways from the people who do not wear contact lenses.

We can see why (B) does not weaken the argument and (A) does.

Hi KarishmaB ,

Request your help. I am torn between A & D:

Below is my understanding of the options:

Okay, so i know that ''wearing contact lenses(CL) CAN cause contact wearers to have dry eyes(DE)''

(A) An inherited condition can cause both weak eyesight and dry eyes.

There can be thousands of reasons, which can cause Dry eyes, and INHERITED CONDITION is one of them
The stimulus is about contact lenses CAN cause dry eyes. How does the option weaken anyway?
Moreover CAN just explains the possibility. IDK, it seems weird.

(B) Physical exertion causes dry eyes in many people who wear contact lenses.
This one shifts the blame to ''PE'', but ''many'' weakens a bit. But this too seems better than A, which mentions nothing about CL wearers.

(D) Most people who wear contact lenses do not have dry eyes.
I am okay to eliminate this option, as CAN denotes possibility, but given an option between A & D, D seems more plausible.


Thanks

When a conclusion says "A causes B," we can weaken it by saying "B causes A" or "C causes both A and B."

Don't worry about the "CAN" used in option (A). It does make us doubt our conclusion and that's all we want. We don't need to PROVE the conclusion. We just have to say that it is possible that A doesn't cause B. Hence option (A) works. It says "an inherited condition can cause both" and that is why they appear together often. Not because one causes the other.

Option (D) slides into Quant category.


Say total 50 people have dry eyes and they buy drops - 10 non contact lens wearers and 40 contact lens wearers.
So pharmacists concluded that contact lenses can cause dry eyes.

(D) Most people who wear contact lenses do not have dry eyes.

Say 200 people wear contact lenses. If only 40 have dry eyes, then most contact lens wearers do not have dry eyes.

But it doesn't weaken our conclusion that contact lenses can cause dry eyes. Perhaps they did in the 40 lens wearers. After all, in the rest of the public, only 10 people have dry eyes.

That is why option (D) doesn't weaken our conclusion.

I discussed this question in a webinar. Check out the video here: https://youtu.be/XCBp62o70Eg

Option (A) doesn't explain why dry eyes would occur only for people with contact lenses but not for people who use glasses perhaps

Notice that the passage doesn't say that people who wear glasses don't experience dry eyes as frequently as people who wear contact lenses. In fact, the passage doesn't say that people who don't wear contact lenses don't experience dry eyes sometimes.

Rather the support for the conclusion is simply "Dry eyes were more frequently experienced by customers who wore contact lenses than by customers who did not wear contact lenses."

So, could be both that people who wear contact lenses experience dry eyes more frequently than all the people who do not wear contact lenses and that people who use glasses experience dry eyes more frequently than all the people who do not use glasses.

So, there's no issue related to people who wear glasses also experiencing dry eyes.

Comparison is between 'Contacts' vs 'Others'.
'Others' includes people with glasses and people with good eyesight.

People with 'contacts' experienced dry eyes more frequently than 'others (glasses + good eyes)'.

(A) An inherited condition can cause both weak eyesight and dry eyes.

This means people with contacts and people with glasses both are likely to have dry eyes more frequently.
The proportion of dry eyes in 'contacts' group will be higher because everyone has weak eyesight there.
The proportion of dry eyes in 'others' group will be lower because only a fraction has weak eyesight there.

Hence (A) works.