A recent research shows that there _____ volcanic activities on the moon and Mars. However, they no longer exist now.
A. had been B. has been C. were D. would be
If she had known what lay ____ for her, she would never have agreed to go.
A. in demand B. in debt
C. in place D. in store
The frozen waterfall has attracted floods of tourists to ____ the natural beauty.
A. cherish B. admire C. observe D. discover
Although he has become wealthy, Mr. Wood remains _______ he used to be, modest and friendly.
A. that B. how C. who D. what
Modern plastics can _________ very high and very low temperatures.
A. stand B. keep C. hold D. remain
---What does PM2.5 mean?
--- It’s a professional ______ indicating the condition of air pollution.
A. number B. mark C. term D. item
