Light in a Frozen Block of Ice Reflects Off the Ice-Air

Light in a frozen block of ice reflects off the ice-air interface at the surface of the block. What phase shift does it undergo?

A) $270 ^ { \circ }$
B) $0 ^ { \circ }$
C) $180 ^ { \circ }$
D) $90 ^ { \circ }$
E) It does not undergo any phase shift.