Randy is sledding down a hill away from his sister, Leslie, who is standing at the top of the hill. At first, Randy moves at a constant speed down a nearly flat part of the hill. Then, suddenly, the hill becomes much steeper, and Randy almost immediately starts moving at a much faster speed.

Which graph shows Randy's position as he moves away from his sister?

W.
X.

Y.
Z.
A.
X
B.
W
C.
Y
D.
Z