Sentence Correction

The idea behind the Personal Long Letter campaign is that a single impassioned constituent may sway a lawmaker's opinion, whereas a half-dozen banded together only causes him alarm.
a) a half-dozen banded together only causes him alarm
b) only alarm is caused by a half-dozen banded together
c) only alarm has been caused by a half-dozen banded together
d) a half-dozen banded together only cause him alarm
e) a half-dozen have caused him only alarm when banded together

[spoiler]OA is A ,is it correct ----- "half-dozen" as a singular noun[/spoiler]

This is my answer concerning the same question on another thread:

Kasia@MasterGMAT wrote:When we talk about amounts and quantities we usually use singular determiners, verbs and pronouns ( even if the noun is PLURAL).

e.g. Where IS that five dollars I lent you?

Twenty miles IS a long way to walk.

A half-dozen eggs IS enough to make a pie.

https://www.beatthegmat.com/sc-singular- ... 13263.html

In my opinion A is right as a half-dozen banded together works as a single unit because of together.