设某种气体的分子速率分布函数为 $f(v)$, 则速率在 ${ }_{v 1}-{ }_{v 2}$ 区间内的分子的平均速率为
A
$\int_{v_1}^{v_2} v f(v) \mathrm{d} v$
B
$v \int_{v_1}^{v_2} v f(v) \mathrm{d} v$
C
$\int_{v_1}^{v_2} v f(v) \mathrm{d} v / \int_{v_1}^{v_2} f(v) \mathrm{d} v$
D
$\int_{v_1}^{v_2} f(v) \mathrm{d} v / \int_0^{\infty} f(v) \mathrm{d} v$
E
F