In time of serious accidents, _______we know some basic things about first aid,we can save lives.

A.whether                           B.until

C.if                                     D.unless

In time of serious accidents, _______ we know some basic things about first aid,we can save lives.

A. whether B. until

C. if D. unless

In time of serious accidents,                we know some basic things about first aid, we can save lives.

       A．whether                                              B．until

       C．if                                                       D．unless

In time of serious accidents, ______ we know some basic things about first aid, we can save lives.

1. A.
   whether
2. B.
   until
3. C.
   if
4. D.
   unless