977.With the effects of global warming, trees______ their leaves much later in the year.

A.are losing

B.have lost

C.lost

D.were losing

978.John seems happy，even though his wife left him and he lost his job.
I think he _______ to put on a brave face.

A.will try

B.has tried

C.tries

D.is trying

977.答案是A;

此句说明现在的情况。

978.答案是D;

考查动词的时态。根据上文语境的暗示，此处应指现阶段的情况，故选D。