The math problem was ________ difficult for me ________ work out. [ ]

On February 25, 2013, Li An, a Chinese director, won the best director again. It is histime to win an “Oscar”.

The research is so designed that once _____ nothing can be done to change it.

I think _________ important to learn English well.

How much does it cost to built the school library?— Fouryuan.