She has a gift for creating an atmosphere for her students ____ allows them to communicate freely with each other.
| A．which | B．where | C．what | D．who |
They were surprised that a child could work out the problem____they themselves couldn't.
| A．once | B．while | C．then | D．if |
.I had been puzzled over the problem for over an hour without any result, _____ all at once thesolution flashed across my mind.
| A．while | B．when | C．then | D．as |
Sally worked late in the evening to finish her report _____ her boss could read it first thing next morning.
| A．because | B．so that | C．before | D．or else |
____much advice I gave him, he did exactly what he wanted to do.
| A．How | B．However | C．Whatever | D．No matter |
Generally speaking, _____ according to the derections , the drug has no side effect.
| A．when to take | B．when taking | C．when to be taken | D．when taken |