The reason why Malaysia Airline Flight370 disappeared remained_____.
| A．look into | B．to be looked into | C．looking into | D．to look into |
The momentJohn will never forget isMr. Smith gave him a lot of valuable advice on how to improve his writing.
| A．that; that | B．that; when | C．when; that | D．when; when |
China first agreedthe six-party talks about North Korea nuclear weapons program.
| A．to attend | B．to join | C．to participate in | D．to take a part |
It is wise of you towell trained workers for untrained ones in your workshop.
| A．substitute | B．change | C．exchange | D．employ |
Many students listened to the lecture, but I wondered how much they.
| A．took out | B．took up | C．took off | D．took in |
The customs office isyour luggage is inspected when you enter or leave a country.
| A．where | B．when | C．why | D．who |