The highway through the rain forest was completed in 1990, ________ some animal species have died off．

1. A.
   by when
2. B.
   since then
3. C.
   since when
4. D.
   by which time

33．The highway through the rain forest was completed in 1990, _________some animal species have died off.

A．by when   B．since then C．since when       D．by which time

The highway through the rain forest was completed in 1990,        some animal species have died off．

A．by when         B．since then          C．since when           D．by which time

33．The highway through the rain forest was completed in 1990, _________some animal species have died off.