求翻译GRE数学题

题目

求翻译GRE数学题
The figure above shows the dimensions of a semicircular cross section 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 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. 5 ft
B. 7 ft
C. 8 ft
D. 9 ft
E. 10 ft
图就是一个半圆,上面标出直径为20ft
我现在看不懂题目.不知道说什么意思.求教高人

答案

直径为20 ft的半圆形交通隧道,行车道在隧道中央,宽12 ft.要求过往车辆距隧道顶部最近处不能少于1 ft,下列多高的车应该被禁止通行?
应该是限高7.2ft
选CDE