Mrs. Wang found his little son ______ with his toys alone when she arrived home. 

I often see my mother ____________ chess with my father after supper. 

---I find the more we study, the more difficulty we have ________ out Maths problems.---Oh, I agree with you. Maths is more and more difficult.

You must eat all kinds of fruits and vegetables         a balanced diet.

Frank asked his father ____ a new bike for him and his father said OK.