No one has yet succeeded in solving the _____ of how life first began.
| A．biology | B．gravity | C．puzzle | D．theory |
______I am concerned，the price of the computer is much too high，so I can’t afford it.
| A．As long as | B．As far as | C．As well as | D．As often as |
Teachers recommend parents_____their children under 12 to ride bicycles to school for safety.
| A．not allow | B．do not allow |
| C．mustn’t allow | D．couldn’t allow |
The following are some popular websites________people can sell goods to each other.
| A．where | B．which | C．when | D．whose |
______ by a snake, she was frightened of it.
| A．Being bitten | B．To bite | C．Having been bitten | D．Biting |
My sister was very unhappy for _______ to the party.
| A．having not been invited | B．not having invited |
| C．having not invited | D．not having been invited |