Multiple Choice question about Proposition Logic

[soopo]

The question is here: http://dl.getdropbox.com/u/175564/logic.png

The correct answer is D.

My attempt to the problem:
Let the beginnings of the sentences be A.
In a, A is B -/-> C is D.
In b, A is B -/-> C is D.
These first two do not make sense.

in c and d, the word "only" suggests exaggeration.
In c, A is B -/-> B must be C.
In d, A is B -/-> A must be C.

Still, I am sure thoroughly why d is the correct answer.
Please, let me know why.

 

[MathematicalPhysicist]

You need modal logic here, for cannot and mustn't, which are modalities.