题目

Since the musician ______ to China, he ______ several concerts in different cities. A. came; had                              B. has come; had             C. came; has had                   D. comes; would have 答案：C生态系统是一个开放的能量消耗系统，用一句话概括其能量流动最基本的特征是 [　　] A．以食物链和食物网为流动渠道 B．单向流动和逐级递减 C．能量传递效率为10％～20％ D．各营养级能量总值构成塔形锥体