It is generally considered that air pollution is ______ danger to ______ nature.
| A．/; a | B．/; the | C．/; / | D．a; / |
If _____ without following the doctor’s instructions, the medicine may have serious side effects.
| A．Taken | B．to be taken | C．Taking | D．being taken |
________ you happen to see Mary, would you please ask her to come here?
| A．Would | B．Could | C．Should | D．Might |
Everyone knows that _______ is dangerous to play with fire, but _______ is difficult is to prevent children from playing with fire.
| A．it, it | B．what, what | C．it, what | D．what, it |
Not until the early years of the 19th century ________ what heat was.
| A．man did know | B．did man knew |
| C．didn't man know | D．did man know |
Tom kept quiet about the accident ________ lose his job.
| A．so not as to | B．so as not to |
| C．so as to not | D．not so as to |