26. A. it is infrequent

=> Dẫn chứng:  When people say something that happens “only once in a blue moon”, they mean that it happens only very rarely, once in a great while.

27. B. May 31st

=> Dẫn chứng:  The moon needs a little more than 29 days to complete its cycle from full moon to full moon. Because every month except February has more than 29 days, every month will have at least one full moon 

28. C. natural activities on Earth can alter the look of the moon

=> Dẫn chứng:  Certain natural phenomena of gigantic proportions can actually change the appearance of the moon from Earth

29. D. been created

=> come into existence: to begin to exist

30. D. polluted air can put a blue coat on the moon

=> Dẫn chứng: The eruption of the Krakatao volcano in 1883 left dust particles in the atmosphere, which clouded the sun and gave the moon a bluish tint.