If industrial pollution continues to deplete the ozone layer, the resu

If industrial pollution continues to deplete the ozone layer, the resulting increase in ultraviolet radiation will endanger human health, causing a rise in the incidence of skin cancer and eye disease, and perhaps even threatening global ecological systems.

As written "ultraviolet radiation will endanger human health, causing ... and perhaps even threatening ...", the sentence seems to present two HOW-aspects of the action "will endanger human health".
It is not correct as "causing a rise in the incidence of skin cancer and eye disease" definitely explains how UR causes bad things to human health, but "threatening global ecological systems" is not. This is an incident of forced parallelism, the second part does not explain the effect of UR of health rather completely different effect.

We need an answer choice that makes parallel another list: "will endanger" and "will threat".

- incorrect for the reason explained above
- correct
- "and" requires us to maintain parallelism: noun ("a possible threat") is not parallel to verb ("will endanger")
- same as A
- same as C

Let’s simplify the sentence:
If pollution continues, the increase will endanger X, causing …, and [threaten] Y.
We can see that X and Y have to be parallel and the verb forms as well. Based on this we can eliminate the options:

(A) and perhaps even threatening – not parallel. Eliminate.
(B) and may even threaten – parallel. Best choice.
(C) and even a possible threat to – not parallel. Eliminate.
(D) as well as possibly threatening – not parallel. Eliminate.
(E) as well as a possible threat to – not parallel. Eliminate.

Hope this helps!
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Notice there is a comma before the underlined part!

That comma means that there are two cases:

1. we have an "and", then the underlined part cannot be parallel with "causing a rise" because "A, and B" pattern is wrong. Therefore, the underlined part must be parallel with "will endanger human health". (actually you can very easily approach this by thinking through the meaning, but I am talking in the scope of grammar)

2. we have an "as well as", then the underlined part must be parallel with "causing a rise" because it follows the format of "A , as well as B".

By the rules above, we can eliminate A, C, E because they are not parallel.

We are left with B and D. However, the meaning in D is illogical: the action of "endanger human health" cannot cause "threaten global ecological systems". Therefore, B correct.


Again, we should always approach SC questions through meaning. But here I am actually working in terms of grammar first just to show you guys the difference between "and" and "as well as".
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(A) and perhaps even threatening Incorrect not parallel to the verb will endanger
(B) and may even threaten Correct
(C) and even a possible threat to Incorrect not parallel to the verb will endanger
(D) as well as possibly threatening Incorrect not parallel to the verb will endanger
(E) as well as a possible threat to Incorrect not parallel to the verb will endanger