在决定买入或卖出期货合约之前，应对合约的（）做全面、准确和谨慎的研究。
A.种类
B.时间
C.价格
D.质量
参考答案：A, C
油层连通图又叫栅状图，它表示油层各方面的岩性变化情况和层间、井间的（）情况。
A、对比
B、连通
C、注采
D、压力变化
--- The fog is so thick that the express way ____ have been closed.
--- You are right. I ____ have thought of that. Now we have to take another road.
A．can; should
B．may; can
C．should; must
D．must; should
