A Cubic Box with Sides of Length a Is Located

A cubic box with sides of length a is located with its center at the origin. A constant electric field is in the + x direction. The side of the cube that has a negative flux is

A) parallel to the y-z plane located at x = -a/2.
B) parallel to the y-z plane located at x = a/2.
C) parallel to the x-z plane located at y = a/2.
D) parallel to the x-z plane located at y = -a/2.