一个英语疑惑一道动词填空____(discuss)this problem is going to take a lon

一个英语疑惑
一道动词填空
____(discuss)this problem is going to take a long.Let's keep our voice down so we will not tell you.老师说这道题是填To discuss,不能填discussing.原因是作为一个目的.
但是我认为这里应该都可以,To discuss 和discussing 都可以做主语
就像It is good for us to read English.=Reading English is good for us .=To read English is good for us.