单项选择题

[听力原文]
M: Mary, How long does it take you to drive home when there is not much traffic
W: Only twenty minutes. But if I can’t leave my office before 11:00 a. m. , it sometimes takes me forty minutes.

How long does it take Mary to drive home when it isn’t rush hour()

A.20 minutes.
B.30 minutes.
C.40 minutes.
D.One hour.