6. As there is less and less coal and oil, scientists are exploring new ways of making use of energy, such as sunlight, wind and water for power and fuel.
| A．primary | B．alternative | C．instant | D．unique |
It was quite ___accident that I met him; he was homeless and ___rags at that time.
| A．by; in | B．on; in | C．at; by | D．in; in |
You should pay a lot of attention to children’s ideas; sometimes they ___be really creative.
| A．must | B．can | C．need | D．should |
He is very humorous, while his brother is rather serious. That is ___their difference lies.
| A．why | B．where | C．how | D．when |
I can still recognize Mr. Johnson, who is standing by the door, even though he looks ___different now.
| A．mostly | B．slightly | C．unlikely | D．finally |
It is only ___our power to carry out ( 执行) the rules, rather than make them.
A. within B. on C without D. off