（8分）小明同学在探究“压力的作用效果与压力大小的关系”时，做了图甲、乙所示的实验，按要求完成下面各题：
蚊、蝇等害虫对DDT产生抗药性的主要原因是 （ ）
________himself in work makes it possible for him to achieve his goal.
