A3. If $F=400$ and $5 \leq a \leq 10$ and $20 \geq b \geq 8$, the difference between the maximum possible value of the expression $\frac{F}{a b}$ and the minimum possible value of this expression is:
(A) 1
(B) 4
(C) 6
(D) 8
(E) 160
A3. If $a=5$ and $b=8$, the value of the expression $\frac{F}{a b}=\frac{400}{5 \cdot 8}=10$. If $a=10$ and $b=20$, the value of the expression $\frac{F}{a b}=\frac{400}{10 \cdot 20}=2$. The difference between 10 and 2 is 8.