Most environmental problems exist because adequate measures for preventing them ____in the past.
------ Do you know the lady ______ is interviewing our headmaster? ------ Yes, she is a journalist from CCTV.
These two books are very interesting. You can choose _________ of them.
He had ________milk but_________ bread for breakfast.
Is there ________you want to say?
---------_________ are you going to visit? --------I am going to visit the Palace Museum.