John _____ knowledge of radio just by staying around the radio station.
| A．caught on | B．worked up | C．took up | D．picked up |
Go back ___ you came from.
| A．until | B．where | C．which | D．when |
The doctor will not perform the operation ___ it is absolutely necessary.
| A．when | B．if | C．for | D．unless |
Difficulties are nothing ___ we are not afraid of them.
| A．for | B．as | C．if | D．whether |
―When do we need to pay the balance ?
― September 30.
| A．In | B．By | C．During | D．Within |
I'd like to go swimming ____ the water is not too cold.
| A．for | B．unless | C．if | D．whether |