题目解析:
以 ingredients 做关键词定位至倒数第二句,有 combining these ingredients:Another generation passed before inventors succeeded in combining these ingredients, by putting the engine on wheels and the wheels on the rails, so as to provide a machine to take the place of the horse.这句话 by 后面就提到了两个事物就是 wheel 和 engine。如果还不确定的话,可以往前看一句,these 说明答案在前句。前句分别说到蒸汽机和马车,所以答案是 B。