Please post the source for any questions you post. Particularly with SC, people really need to know the source to make sure they want to study the question - a lot of sources for SC aren't good enough to warrant study. Thanks!

For the first one, the sentence starts in present tense ("appears") and indicates an "always true" type of situation. When the sentence then discusses why this is "always true," the appropriate tense is present perfect, which is used for something that started in the past but is still true or still going on today. "Were" is simple past, which means that event is over.

That eliminates A, C, and D. And "increasing difficulties" lets us eliminate E - the difficulties aren't continuing to increase over time.

For the second one, the original sentence indicates a comparison, but it is comparing "transplants" and "all patients." That isn't a logical comparison - you'd have to compare something like "transplants between identical twins" vs. "transplants between non-identical twins." A is eliminated.

B gives the wrong meaning (besides) and is not logical (it seems to say only those "identical twins with the same genetic endowment" but all identical twins are genetically the same). D has the same problem. E also tries to make a comparison ("other thhan transplants... all patients") but the comparison isn't logical.

Who vs. whose - one is the subject form and one is the possessive form. So, "whose genetic endowment" is okay b/c this is possessive. And "who have the same..." is fine too b/c this is a subject. Both versions are fine (as far as this issue is concerned).