SC - Alarm

[mygmat800]

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

[Guest]

Well..my question here is - In each the following cases, the subject is plural or singular?

a half-dozen constituents only cause/causes him alarm

AND

a half-dozen constituents banded together only cause/causes him alarm.

[mygmat800]

Source: GMATClub Challenges.