题目

保护我国自然资源的合理措施是（   ） A．彻底消灭有益动物的天敌                       B．控制人口增长 C．大量引进外种并及时推广                          D．大量开荒和围湖造田 答案：BHe studied very hard all the time because he didn't want to _____in his studies. A. fall behind　 　B. be left behind　 C. left behind　 　 D. Both A and B