$35 \cdot 18$ In a dark room, there are several red, green, blue, yellow, and white socks. To ensure that 10 pairs of socks (two socks of the same color make a pair) are taken out from the room, the minimum number of socks that should be taken out is
(A) 23.
(B) 24.
(C) 25.
(D) 26.
(China Hubei Huanggang Junior High School Mathematics Competition, 1990)
［Solution］If 23 socks are taken out, at most 5 of them can be single (five colors), and the remaining 18 can form 9 pairs of socks.
5 single socks, taking 1 more can form another pair.
Thus, when 24 socks are taken out, it can be guaranteed to form 10 pairs of socks.
Therefore, the answer is $(B)$.
Note This conclusion can be generalized as:
For $m$ different colored socks, to ensure forming $n$ pairs, at least $2 n+m-1$ socks need to be taken.