Soul music first rose to prominence in the United States in the 1960s, when artists were combining subtle African rhythms with gospel harmonies and themes, as well as influences from the newly emergent genre of rock-and-roll.

when artists were combining
during which many artists were combining
at which point artists began to combine
when artists began to combine
when artists who began to combine

OA D
Official Explanation:
[spoiler]The content of the sentence concerns a point in time when soul music first became popular, making the use of the word began and simple past tense necessary.
A. The use of past progressive tense in this option makes it sound as if the activities described in the sentence continued throughout the 1960s; the option does not correctly discuss a starting point for soul music.
B. During which is redundant and unnecessary.
C. The phrase at which point is wordy and unnecessary.
D. This option correctly identifies the need to use simple past tense to identify a specific point in time when soul music began to be popular.
E. The word who in the option makes the sentence incomplete.
The correct answer is D.[/spoiler]
My question is:
Can we find any parallelism between began to combine and as well as influences ? I think that they should be parallel, but it seems to me that they are not. Or here it is not needed?