请教一道GWD数学题~

only红番茄

The figure above shows the dimensions of a semicircular cross section横截面 of a one-way tunnel.  The single traffic lane is 12 feet wide and is equidistant from the sides of the tunnel.  If vehicles must clear the top of the tunnel by at least ½ foot when they are inside the traffic lane, what should be the limit on the height of vehicles that are allowed to use the tunnel?

A.    5½ ft
B.    7½ ft
C.    8 ½ ft
D.    9½ ft
E.    10 ft
图中是一个半圆，直径处标有20ft

only红番茄

不知道怎么插入图片啦...