### Thanks for moving this thread to "Discrete Mathematics, Set Theory and Logic" ###

For the following question, I think the answer should be (d). The yellow highlighted area shows my working steps.

But someone told me that it shoud be (e) instead. I wonder if there is any mistake inT。Sand your advice will be much appreicated.

http://img697.imageshack.us/img697/1230/cma4205.png