gmat_perfect wrote:Whereas in mammals the tiny tubes that convey nutrients to bone cells are arrayed in parallel lines, in birds the tubes form a random pattern.

(A) Whereas in mammals the tiny tubes that convey nutrients to bone cells are arrayed in parallel lines, in birds the tubes

(B) Whereas the tiny tubes for the conveying of nutrients to bone cells are arrayed in mammals in parallel lines, birds have tubes that

(C) Unlike mammals, where the tiny tubes for conveying nutrients to bone cells are arrayed in parallel lines, birds' tubes

(D) Unlike mammals, in whom the tiny tubes that convey nutrients to bone cells are arrayed in parallel lines, the tubes in birds

(E) Unlike the tiny tubes that convey nutrients to bone cells, which in mammals are arrayed in parallel lines, in birds the tubes

This question is testing parallelism. Choice B compares tiny tubes to birds, as does E. Choices C and D compare mammals and birds' tubes or the tubes in birds. Even though it sounds a little awkward, choice A correctly parallels "in mammals the tiny tubes..." with "in birds the tubes..." That makes A the best choice.

As Andrea pointed out, this tests parallelism in comparisons. C and D can be eliminated because they compare mammals to tubes: "Unlike mammals,...tubes"

E offers an unparallel comparison as well: "Unlike the tiny tubes" is not parallel to "in birds the tubes".

Another defect of E is the misplacement of the modfier "which in mammals are arrayed in parallel lines". Like all modifiers that begin with a relative pronoun (which, who, whom, whose, when, where, that...), this is a noun modifier. As a result, this modifier should be placed next to its intended target (the tiny tubes), but it is too far from its target.

The official answer is A. A more detailed solution is available at GMATPrep Question 2172.

-Patrick