If the resistance doubles while the voltage stays the same, what effect will that have on the power?
a. it will increase by a factor of 2.
b. it will decrease by a factor of 2.
c. it will increase by a factor of 4.
d. it will decrease by a factor of 4.

The best formula to use for this question is

             Power = (voltage)² / R

So      Original power = (voltage)² / (original R)
and
           New power  =  (same voltage)² / (2 x original R)

That's equal to            (1/2) (voltage)² / (original R)

which is equal to    (1/2) (original power) .

So I guess the power decreases by half, and that's choice-B .