(1) Frogs eat pests which _____ crops. (2) We need to ______ right from wrong. (3) Students in this university come from ______ backgrounds. defense n. 防卫,防卫物;vt. 谋划抵御; destroy vt. 杀死; 破坏,摧毁; 消灭,歼灭(敌人); 使失败; discourage vt. 劝阻; 阻碍; 使气馁; 使沮丧; distinguish vi. 区分,辨别;vt. 区分,辨别,分清; 辨别出,识别; diverse adj. 不同的,多种多样的; dozen n. (一) 打,十二个;adj. 一打的;