I find _____ difficult for us to master English if we don’t work hard.

I find _____ difficult for us to master English if we don’t work hard.

1. A.
   that
2. B.
   it
3. C.
   this
4. D.
   what

 I find _____ difficult for us to master English if we don’t work hard.

A. that           B. it          C. this         D. what

I find _____ difficult for us to master English if we don’t work hard.