______ there are living creatures in the outer space as those on Earth hasn’t been proved so far.

______ there are living creatures in the outer space as those on Earth hasn’t been proved so far.

A．When            B．How              C．If                D．Whether

________there are living creatures in the outer space as those on Earth hasn't been proved until now.

A．How  B．When

C．Where  D．Whether

___ there are living creatures in the outer space as those on Earth hasn’t been proved until now.

1. A.
   How
2. B.
   When
3. C.
   Where
4. D.
   Whether

______ there are living creatures in the outer space as those on Earth hasn’t been proved so far.

1. A.
   When
2. B.
   How
3. C.
   If
4. D.
   Whether