It seems that the students devote much time and energy _____ a high score in the exam.
Ask John ______ here at 7 pm tomorrow.
The air in Beijing is getting much ______ now than a few years ago.
He has two pets. One is a dog, ______ is a cat.
— Will you stay for lunch? — Sorry, I ______. My brother is coming to see me.
I phoned you at six yesterday evening, ______ nobody answered.