21. The teacher divided class ________four groups.
22. He provide us ________a lot of useful information.
23. The ground is covered ________trash. You should clear ________all the trash.
24. Raw sewage is pumped directly ________the sea.
25. They are trying to save the animals ________extinction.
26. The world will end ________like a second-hand junk-yard.
27. I am looking forward ________hearing from you.
28. Don’t throw trash ________the water
