Amazing members of this amazing glub, need your help again.

Qn:

Unlike transplants between identical twins, whose genetic endowment is the same, all patients receiving hearts or other organs must take anti-rejection drugs for the rest of their lives.

(A) Unlike transplants between identical twins, whose genetic endowment is the same
(B) Besides transplants involving identical twins with the same genetic endowment
(C) Unless the transplant involves identical twins who have the same genetic endowment
(D) Aside from a transplant between identical twins with the same genetic endowment
(E) Other than transplants between identical twins, whose genetic endowment is the same

Official answer is C. If you are testing your SC skills on this question, read no further until you have chosen your answer.

It took me a lot of time to solve this.

I try to attack a gmat SC question, by looking for the differences in the answer

Using the differneces

"the same genetic endowment" is better than "genetic endowment is the same" - So A & E are not first preferred choices

"with the same" in B & D is not right - So B & D are totally gone.

So C looks good. (In a exam, with no time at hand, i will go for C) But have to find out strong reasons to take A, E out. The similarities are "Whose" , "transplants"

"whose" vs "who"
"transplants" vs "transplant"
If i knew this, i would have cracked this under 1 min.

Please help.