Is the Ans C ???

it is D... the issue here is not parallelism. That is just a matter of form which is imp on the gmat. But here substance takes precedence over form. What is the sentence saying about the cells?

That they adhere to each other as they beat together but at the same time they adopt postures which are exclusive of each other... These are contradictory things - there is a contrast between opposites here. You need a marker such as "yet" to express the contrast. D does that.

shreeman wrote:18.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.
(A) beating in unison and adopting
(B) they beat in unison while adopting
(C) beat in unison, and adopt
(D) beating in unison yet adopting
(E) even though they beat in unison and adopt

OA is D

why D is incorrect. Isn't it following the ll'ism - adhere, beat, and adopt.

sorry i incorrectly wrote why D is incorrect but i mean to say why C is incorrect. D is the right ans.

shreeman wrote:
sorry i incorrectly wrote why D is incorrect but i mean to say why C is incorrect. D is the right ans.

There are two things important here -
1. Use of and is incorrect. beating in unison and adopting specialized orientations exclusive of one another are two logically opposite ideas and must be joined by an appropriate conjunction (like yet) to bring out this opposition.
2. beating in unison yet adopting specialized orientations exclusive of one another is a comma + ing modifier modifying the subject of the preceding clause.