是第二套的第六题。图是一个半圆，半圆直径处标了20ft。可题中明明说的是12，因此一头雾水，望侠士解答~~~

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 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
Answer: B

半圆是隧道，直径就是20feet

single traffic lane 是轨道，宽度是12feet，位于隧道的正中央