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31. A

-> paragraph 1, sentence 1, line1 : 

Evaporation and recondensation of water entail an important step in purification called distillation.

-> paragraph 2, sentence 1, line 1 :

Water can be purified by distillation or other methods.

32. D

-> paragraph 1, sentence 1, line 1 & 2 :

During evaporation, ............. , forming sediment.

-> crystallize = solidify . 

33. A

34. B

35. D