题干
____________ people went to the concert which was held in People’s Square the night of May Day.
A：Three thousand, i
B：Thousands, o
C：Many thousand, i
D：Thousands of, o
答案(点此获取答案解析）
D
，据市场分析每辆客车营运的总利润
y(单位：万元)与营运年数x(x∈N)为二次函数关系(如图)，则每辆客车营运多少年，其营运的年平均利润最大（ ）