题目

Today Chinese is becoming more and more popular．It       in many schools around the world． A．teaches                 B．is teaching            C．has taught             D．is taught 答案：D(襄樊四中模拟)已知为等差数列，若，且它的前n项和有最大值，那么当取得最小正值时，n等于 [　　] A．11 B．20 C．19 D．21