We must ___ on our study..
| A．concentrate | B．concentrate at | C．concentrate on | D．concentrate with |
After visiting Harbin, most of the foreign friends said they would never forget the time ________ they had spent.
| A．that | B．what |
| C．when | D．at which |
Eleven people got killed in the accident, ________ the identities haven't been available.
| A．whose | B．of whom |
| C．to whom | D．with whom |
It's said to be a case of murder ________ the children were used by adults.
| A．when | B．where |
| C．which | D．that |
Naturally a smile ________ the eyes participate is extremely communicative.
| A．to which | B．in which |
| C．at which | D．with which |
With hundreds of attractions, Beijing is a city ________ makes a lasting impression on its visitors.
| A．which | B．where |
| C．what | D．when |