题目

下列实例，根据其应用的原理在实质上的区别进行分类，最合理的是 () ①摘去棉花的顶芽，促进产量的进步②给糖尿病患者注射胰岛素，医治糖尿病 ③马戏团训练小狗表演杂技④在未成熟的香蕉上喷洒大量低浓度乙烯，促进其尽快成熟 A . ①④和②③ B . ②④和①③ C . ①②④和③ D . ②和①③④ 答案：C He spent as much time as he could _____ the child. A. teaching       B. to teach    C. teach       D. for teaching