be helpful in doing 怎么造句

be helpful in doing 怎么造句 be helpful in be helpful to be helpful for 三者区别即怎么用 be occupied in doing be behind in doing be expert at doing 还是in doing 1.be useful to_______ 2.be helpful to______3.be important to______4.be necessary to______填doing或do be in charge of doing be interested in doing sth be busy in doing sth be long in doing sth be behind in doing sth be sophisticated in doing sth be busy doing 和 be busy in doing 的区别? be successful in doing in可省吗 it is helpful to do sth it is helpful for doing sth it is helpful in doing sth 那个正确?Doing exercise is helpful ________ healthy. A. to keep B keeping C.keep D in keeping 哪个正确?选哪个 谢了如上 2个问题求 详解， be worth doing sth .doing前面加in么? be interested in为什么加doing [急]be interested in后是加doing么?