This problem may lead to more serious ones if ____ unsolved.
| A．making | B．remained | C．keeping | D．left |
The thought _____ they could cross the whole continent was exciting.
| A．that | B．what | C．which | D．why |
We watched, amazed as fire _____ on the outside of the spaceship as the earth’s gravity increased.
| A．broke up | B．broke out | C．broke in | D．broke away |
Walking does need a bit of practice _____ gravity has changed.
| A．as if | B．in their turn | C．now that | D．as a result of |
They multiplied and filled the oceans and seas with oxygen, _____ encouraged the later development of early shellfish and all sorts of fish.
| A．that | B．where | C．when | D．which |
_____ is even more important is that as the earth cooled down, water began to appear on its surface.
| A．As | B．That | C．What | D．Where |