Can any one reason why OA is D?
The participial phrase concept is being tested here. The sentence is in present tense here, and therefore the tense should be mantained throughout. Thus the answer choice should start with 'beating'. Always remember, a participial phrase will always be set off by a comma.
Thus this leaves us with answer choices A and D. Of the two, D sets up the contrast between beating in unison and adopting specialized orientations nicely. Thus D.
Notes on participles
Participles are verbs that usually end either with ing, or ed. They act as adjectives. Present participles end with â€˜ingâ€™. Present participles are frequently tested in the GMAT. Past particples end with -t, -ed, -d, -en or â€“n as in dealt, asked, saved, eaten or seen. A participial phrase is usually set off by commas. A participial phrase should be as close to the nouns and pronouns they modify as possible, and those nouns and pronouns should be clearly stated.