Li Ming wants to buy some gifts for his mother and cousin in China. He goes to a shop. He buys two dresses for his cousin and some dishes for his mother. Now he needs some boxes. Jenny gives the boxes to Li Ming. Li Ming says thanks to Jenny and writes letters to his mother and cousin. Then Li Ming goes to the post office and buys four stamps. He thinks that his mother and cousin will like the gifts. He walks back to his house. He feels happy.

（2015•临沂）近年来，抗生素的滥用致使很多致病细菌具有一定的耐药性．根据达尔文的进化论分析，耐药细菌越来越多的主要原因是（　　）