.My parents would have come to see you if it had been possible, but they ________ so busy.
He asked where she ____, but she didn’t respond.
The teacher ___________his students to work hard.
After the age of 50 people often _____________ their individual ambitions.
He is a key ____________ in the murder so we must protect him from all kinds of danger.
Russia is _________ on the north of China.