表示每增加一单位可变投入所增加的产量是( )
A．总产量 B．平均产量C．边际产量 D．单位产量
参考答案：C
等差数列{an}中a4=5，则其前7项和S7的值为______．
It is sleeping late in the morning that _____ being late for work.[ ]
A. devotes to
B. sticks to
C. refers to
D. leads to
