It is important to know about the cultural differences that        cause problems.

 As to the question of whether we will sell our house or who we shall sell the house to, you ____ have answer by tomorrow.

– Must I stay here with you?-- No, you ______. You may go home, but you _____ go to the net bar(网吧).

    --- Whose backpack is this?--- It        be Mary’s. She is looking for hers.

We should spend as much time as we _______ over our lessons.