[求助] Help on GWD 2 gmat-M

loxxx

It's not quite convenient for me to show the figure here so I omit it (the figure is basically a semicircular 20 feet diameter):

Q6:

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

The official answer is B. Can somebody explain that? Thanks.

Btw, the official answer to Q5 is "D/A". This is pretty confusing to me. Why is it?

Q5 (again I omit the figure):

In the figure, points A, B, C, D, and E lie on a line.  A is on both circles, B is the center of the
smaller circle, C is the center of the larger circle, D is on the smaller
circle, and E is on the larger circle. 
What is the area of the region inside the larger circle and outside the
smaller circle?

        

(1)
              AB = 3 and BC =2

        

(2)
              CD =1 and DE = 4

        

                  

        

A. Statement (1) ALONE
is sufficient, but statement (2) alone is not sufficient.

        

B. Statement (2) ALONE
is sufficient, but statement (1) alone is not sufficient.

        

C. BOTH statements
TOGETHER are sufficient, but NEITHER statement ALONE is sufficient.

        

D. EACH statement
ALONE is sufficient.

        

E. Statements (1) and (2) TOGETHER are NOT
sufficient.