SAT Writing Question Please help

Test Preparation SAT Preparation
1

Aeschylus designed costumes, trained his choruses, and probably acted in most of his own plays, (as was) the usual practice among ancient Greek dramatists.

a. as was
b. as being
c. this was
d. these were
e. it was

Why is the correct answer “as was”? And how to distinguish the occasion in which you use “as was/were” and “as being”???

2

“Being” is almost always wrong (it’s usually only right if some parallelism requires it)
“as was/were” is a subject-verb agreement issue–the subject in this case is the singular “the usual practice.”
@orangubum