资本主义经济危机周期性爆发的物质基础是( )
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.