1.A peculiar feature of the embryonic mammalian circulatory system is that in the area of the heart the cells adhere to one another, beating in unison and adopting specialized orientations exclusive of one another.
Thoughts during dream bestow me inspiration. Now I get through the answer.
1. In GMAT SC, the most important thing is to express the original meaning precisely and clearly.
2. In this sentence, "beating in unison and adopting" is adverbal modifier to modify the "adhere to". C just tamper the modifier with an verb clause, just distort the original meaning of concomitant modifier.
3. beating unison, unison means "concord". while adopt A exclusive of one another, means that A is not similar between one cell and another. Thus you can not join the turn meaning with "and". As a result, D is the right answer.