Experts are trying to work out a plan to deal with the problems caused by global warming ______ it is too late.

Experts are trying to work out a plan to deal with the problems caused by global warming ______ it is too late.

A．when            B．if                C．before            D．until

Experts are trying to work out a plan to deal with the problems caused by global warming _________ it is too late.

1. A.
   if
2. B.
   before
3. C.
   when
4. D.
   until