e up to sth.胜任某事
be busying doing 忙于做某事(正在进行
如:
What are you up to？你在忙什么？
What are these naughty boys up to?这些淘气的孩子们在干什么？
It's up to me to get the four of us moving.由我来负责我们四个人的行动
②be busy后面跟doing，doing sth是现在分词短语作伴随状语，be busy可以看成是复合动词，be busy doing sth是固定搭配，意思是“忙于做某事”，be busy doing sth侧重于动作。