There will still be lots of challenges if we are to ______ garbage in a short time．（ ）
| A． |
clarify |
B． |
justify |
C． |
satisfy |
D． |
classify |
The little problems ______ we meet in our daily lives may be inspirations for great inventions.
| A． |
that |
B． |
as |
C． |
where |
D． |
when |
The national park has a large collection of wildlife, _________ from butterflies to elephants.
| A． |
ranging |
B． |
range |
C． |
to range |
D． |
ranged |
In the 1950s in the USA, most families had just one phone at home, and wireless phones _______ yet.
| A． |
haven't invented |
B． |
haven't been invented |
| C． |
hadn't invented |
D． |
hadn't been invented |
If you don't understand something, you may research, study, and talk to other people _______ you figure it out.
| A． |
because |
B． |
though |
C． |
until |
D． |
since |
Many airlines now allow passengers to print their boarding passes online ______ their valuable time.
| A． |
save |
B． |
saving |
| C． |
to save |
D． |
saved |