题目

A good teacher always leaves his students enough time to think and voice their opinions in class，_________ sometimes he may not complete his teaching task．A．even if        B．in case       C．as long as      D．as if 答案：A“可燃冰”的主要成分是甲烷与水分子的结晶水合物(CH4·nH2O)．其晶体类型是A.离子晶体B.分子晶体C.原子晶体D.金属晶体