$32 \cdot 26$ Let $n$ be a natural number between 100 and 200, then the number of $n$ that satisfies $7 n+2$ is a multiple of 5 is
(A) 10.
(B) 11.
(C) 20.
(D) 21.
(2nd "Five Sheep Cup" Junior High School Mathematics Competition, 1990)
[Solution] If $7n+2$ is a multiple of 5, then its unit digit can only be 0 or 5, thus the unit digit of $7n$ must be 8 or 3, which means the unit digit of $n$ can only be 4 or 9.
That is, $n$ can only be $104, 109, 114, 119, \cdots, 194, 199$, a total of 20 numbers. Therefore, the answer is (D).