He achieved his aim __B__ A.by hard working B.by hard work 为什么选B

by为介词后面要加上名词或者是V-ing .
但是如果选A的话,应该是by working hard.这里的hard是副词.by hard work.这里的hard 是形容词.
是位置错了!.A选项的working是动名词,副词应放在其后面!