题目

When I entered the step-classroom(阶梯教室), the lecture _________ and the people _________ attentively. A . began, listened B . was begun, was listening C . had begun, listened D . had begun, were listening 答案：D7．设等比数列{an}前n项和为Sn，若S10：S5=1：2，则$\frac{{{S_5}+{S_{10}}+{S_{15}}}}{{{S_{10}}-{S_5}}}$=$-\frac{9}{2}$．