Air pollution will remain a problem for a long time， ______ we have done a lot to solve it．（ ）
| A． |
though |
B． |
since |
C． |
unless |
D． |
if |
John________ us when he is free．（ ）
| A． |
joins |
B． |
join |
C． |
will join |
D． |
joined |
It's rather cold here． You'd better ________ your coat．（ ）
| A． |
put away |
B． |
not put on |
| C． |
not take off |
D． |
take off |
I'm afraid we can't work out the math problem________the teacher helps us．
That's true． It's too difficult．（ ）
| A． |
if |
B． |
unless |
C． |
because |
D． |
as soon as |
May I use your computer？
________． I'm working on it．（ ）
| A． |
Glad to hear that |
B． |
It doesn't matter |
| C． |
That's all right |
D． |
I'm sorry |
Basketball________ in 1891． Then in 1936 in Berlin， it became an event at the Olympics．（ ）
| A． |
plays |
B． |
is played |
C． |
was played |
D． |
played |