Normally it takes a week for a cake to become moldy in a refrigerator.

My take on why D is incorrect:

Conclusion has the word "must". So the correct option should say that in any case (100% of all times), the cake has been in the refrigerator for atleast a week.

Option D says 95% refrigerators can keep a cake fresh for a week. This means 5% refrigerators fail to keep a cake fresh for a week. This 5% does not bode well with the "must" condition and hence Option D is incorrect.

Normally it takes a week for a cake to become moldy in a refrigerator. The cake in Alex's refrigerator is moldy. Therefore, the cake in Alex's refrigerator must be at least a week old.

Which of the following, if true, strengthens the conclusion?


Here author is claiming that the moldy cake was there in refrigerator at least 1 week means that the cake was not moldy initially. Option C correctly states that.


A. Alex's refrigerator has not been cleaned in two years.

B. A blown fuse in Alex's building has deprived his refrigerator of electricity for the past five days.

C. The cake had just been baked when it was placed in Alex's refrigerator; it has remained there ever since.

D. A recent study demonstrated that 95% of refrigerators currently in use will keep a cake fresh for one week.

E. The cake was baked on a Tuesday.

Why isn't B the answer? B says that fridge is out for past 5 days. The cake is already moldy. It takes a week to mold the cake in fridge. So, 5 days + 7 days(molding)= 12 days

If anything, this answer makes it possible that the cake molded in fewer than 7 days.
Let's say, your fridge is of standard quality, and it was working in a standard setting without any failure. In this case your cake should be fine for a week without molding.
But if your fridge had no power for 5 days, maybe the cake went in just 3 days.

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