If the current and the resistance in a circuit both double, what will happen to the voltage?
a. stays the same
b. doubles
c. halves
d. quadruples
e. decreases by a factor of 4

Voltage is the dependent variable.

You can double the resistance by wiring more resistors into a
series circuit, or removing some from a parallel circuit.

At that point, the only way you can double the current ... compared to
the original current before you started messing with things ... is to
increase the voltage to 4 times what it was originally.  If you don't
do that, then the current in the circuit will not double.

Efficiency is an act of doing something