$35 \cdot 15$ Four students go to a store to buy either a brush pen or a pencil, each buying only one pen, and at least one person buys a pencil. Then the possible ways of buying are
(A) 4 kinds.
(B) 5 kinds.
(C) 15 kinds.
(D) 16 kinds.
(2nd "Five Sheep Cup" Junior High School Mathematics Competition, 1990)
[Solution] Each person has two possible choices: buying a brush pen or a pencil. For four people, there are $2 \cdot 2 \cdot 2 \cdot 2 = 16$ choices. However, one of these choices is where everyone buys a brush pen, which does not meet the problem's requirements, so it should be subtracted. Therefore, there are $16 - 1 = 15$ ways to buy.
Thus, the answer is (C).