A dielectric slab of thickness d is inserted in a parallel plate capacitor whose negetive plate is at x=0 and positive plate is at x=3d. The slab is equidistant from the plates. The capacitor is given some charge by connecting it to a batterey. As x goes from 0 to 3d,
A. the magnitude of electric field remains the same
B. the direction of electric field remains the same
C. the electric potential increases continuously
D. the electric potential increases at first, then decreases and again increases
