how to treat this kind of question

[NSNguyen]

If A, then B.
If B, then C.
If C, then D.
If all of the statements above are true, which of the following must also be true?
(A) If D, then A.
(B) If not B, then not C.
(C) If not D, then not A.
(D) If D, then E.
(E) If not A, then not D.

[abhishingwekar]

In case you are stuck up in this kind of question look into answer choices....

I am not sure, but I think the answer is E.What's the OA??

[Ian Stewart]

Have never seen a real GMAT question like this- this is a question in mathematical logic. Whenever you know "If A, then B", you can always conclude the contrapositive is true: "If not B, then not A". You cannot conclude the 'converse' ("If B, then A"), or the 'inverse' ('If not A, then not B').

You can try this with a mathematical statement:

A could be 'x = 2', and B could be 'x^2 = 4'.

Then If A is true, B is true ('If A, then B'). But, if B is true, A is not necessarily true; x could be -2. That is, the converse is not a legitimate deduction to make. The contrapositive is always a legitimate deduction: if B is false, then A must be false. That is, if x^2 is not equal to 4, then it's certainly true that x is not equal to 2.

Back to the question: If A implies B, B implies C, and C implies D, then A implies D. We know "If A, then D". The contrapositive of this must also be true: "If not D, then not A". So the answer is C.

It can be useful to understand contrapositives for CR questions, but you will never see a question presented like the above on the GMAT; instead you'll see a problem with some kind of argument about a real-world situation.

[newera]

I think the answer is C.

The stimulus tells us
A-->B
B-->C
C-->D

which is the same as A-->D. In logic, we cannot reverse this to say that D-->A, this is not always the case. However, we can say that Not D-->Not A.
Its the only trick I remember from that class!

[hydgrr]

Is the answer not B??

[anju]

Thanks for the detailed explanation Ian. I encountered this question earlier and was wondering why the right ans is C.

Thanks

[john83.amar]

I will Go with option C in this case