1. 122 For the formula $C=\frac{e n}{R+n r}$, when $e, R, r$ are constant, if $n$ increases, then $C$
(A) decreases.
(B) increases.
(C) remains unchanged.
(D) increases then decreases.
(E) decreases then increases.
(1st American High School Mathematics Examination, 1950)
[Solution] Transform $C$ into $C=\frac{e}{\frac{R}{n}+r}$. When $n$ increases, the denominator decreases, so $C$ increases. Therefore, the answer is $(B)$.