86. In the following spring , the seeds should be knocked _______ the seed-heads and sown .
He stumbled(绊倒)on the _______ ground and fell flat on his face.
Many students today suffer from ________ problems, since they are under too much pressure.
________ energy is generated from natural resources—such as sunlight, wind, rain, tides and so on.
Effective measures have been taken to prevent _______pollution.
-- I’m thinking of the test tomorrow. I’m afraid I can’t pass this time.-- ______! I’m sure you’ll make it.