题目

Billy Peck was a farm duck whose biggest dream was to become a tightrope walker (走钢丝者). Every day he __16__ hours practicing on a rope, encouraged by his best __17__， Artie Quack. One day, a sheep arrived at the __18__. Soon after the duck finished their tightrope practice, the sheep __19__ and began praising them. He said they had done __20__ work, and he believed that they could cross any precipice (悬崖) on that rope. This left Billy feeling very __21__. But Artie said that he didn’t notice __22__ real improvement. A few days later, they went to a precipice. The space be长期使用某种杀虫剂来防治害虫，效果会越来越差．害虫产生抗药性的原因是（　　） A、害虫对杀虫剂选择的结果B、害虫定向变异的结果C、杀虫剂对害虫选择的结果D、药量太小或药物失效