$35 \cdot 19$ In a drawer in a dark room, there are 100 red socks, 80 green socks, 60 blue socks, and 40 black socks. A young man picks a sock from the drawer each time, but he cannot see the color of the sock he picks, to ensure that there are at least 10 pairs of socks, the minimum number of socks he must pick is
(A) 21 .
(B) 23 .
(C) 24 .
(D) 30 .
(E) 50 .
(37th American High School Mathematics Examination, 1986)
[Solution] Picking 5 socks can ensure that at least one pair of socks is formed. After taking out this pair, 3 socks remain.
Taking another 2 socks from the remaining 3 can form another pair of socks.
Continuing this process, it is not difficult to see that when
$$
(3+2)+\underbrace{2+2+\cdots+2}_{9 \uparrow}=3+10 \cdot 2=23
$$

socks are taken out, it can ensure that 10 pairs of socks are formed.
Therefore, the answer is (B).