下图表示甲、乙、丙三个种群数量变化，试根据图回答有关问题：
丙种群在C点时，种群的年龄组成如图中的（）
A.
B.
C.
参考答案：B
Outdoor activities are as much a part of childhood for this generation of Chinese children as they __________ for their parents and grandparents.
A．are
B．will be
C．were
D．have been
如图所示，在水平地面上平铺n块砖，每块砖的质量为m，厚度为h，如将砖一块一块地叠放 起来，至少需要做多少功？
