a 的用法是
find it adj（形容词做宾补） + to do (真正的宾语）
B的用法是：
did you find that it was hard to learn english
find后面是跟宾语从句
而这个宾语从句又是it is adj to do sth的主语从句that可省略？宾语从句的that是可以省略的