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，

Question

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， 这是基础训练的一道题 后边的答案说选D 不过我不懂 求解答 不知道 it is helpful in doing sth 对吗？

Answer 1

It's helpful to do sth 是正确的.It 是形式主语 真正的主语用 to do sth
It is helpful to be informed .
be helpful to sth to 是介词 ,后应该跟 动词的ing 形式所以A BC都不对
be helpful in （doing ）sth 表示在某方面有帮助
Doing exercise is helpful ___in keeping_____ healthy