在用比较法确定经济合理剥采比的最常见的三种方法为，（）、（）和（）。
参考答案：原矿成本比较法；金属成本比较法；储量盈利比较法
We students，________ the future of our motherland，should devote all our ________ to our studies to make our dreams come true.[ ]
A．compared with; powers
B．comparing to; strengths
C．are compared to; energies
D．compared to; energies
I just don’t understand ________ that prevents so many children at school from being as happy as one
might expect.[ ]
A. what it is
B. what it does
C. what is it
D. why is it
