5.23 Given the graph of the quadratic function $y=a x^{2}+b x+c$ as shown in the figure, then among the following 6 algebraic expressions $a b, a c, a+b+c, a-$ $b+c, 2 a+b, 2 a-b$, the number of expressions whose value is positive is
(A) 2.
(B) 3.
(C) 4.
(D) More than 4.
(China Junior High School Mathematics League, 1989)
[Solution] If the graph opens downwards, then $a<0$, $2a+b<0$, and $a-b+c<0$. Therefore, out of the 6 algebraic expressions, only 2 are positive. Hence, the answer is $(A)$.