________ there are living creatures in the outer space as those on Earth hasn't been proved until now.
#30306
A
How
B
When
C
Where
D
Whether
推荐题目
It's really very dangerous.One more step，________ the baby will fall into the well.
#30469
A
or
B
so
C
but
D
and
.Look out! Don’t get too close to the house ______ the roof is under repair.
#33022
A
whose
B
which
C
of which
D
what
He doesn’t have the habit of planning his travel expense, so he won’t know what to do _______ the money runs out.
#34286
A
once
B
before
C
unless
D
though
Scientists say it may be five years ______ it is possible to test this medicine on human patients．
#31988
A
since
B
until
C
before
D
when
______the Internet is of great help, I don’t think it’s a good idea to spend too much time on it.
#79821
A
If
B
While
C
Because
D
As