The Potential (Relative to Infinity) at the Midpoint of a Square

Multiple Choice

The potential (relative to infinity) at the midpoint of a square is 3.0 V when a point charge of +Q is located at one of the corners of the square. What is the potential (relative to infinity) at the center
When each of the other corners is also contains a point charge of

A) 12 V
B) 9.0 V
C) 0 V
D) 3.0 V