Sentence Correction

pls discss
OA : B

IMO C.
Not sure..
Actually none of the choices come out clearly as winner. Further,
I am troubled by the usage of 'it' in many choices including C.

So I will withhold my explanation and watch others.

efficiency of is probably the right idiom...

let us think, if the testmaker corrected the idiom error..then how come B is a better choice than A

if i could neglect the idiom error...which i should not but if i do

A & B are comes up

With A there is one problem...

thereby boosting consumption...modifiers the previous clause....
which means that demand will boost consumption.....

Since
B is the given right answer......
i guess authors contended that increasing supply and lowering prices boost consumption......but i guess they have wrongly used WHICH....
WHICH is illogically referring to lowering prices.
or i am missing any rule

X and Y, which boost z.

does which refer to both X and Y ???

Any ideas ?