16. The World Health Organization gave a warning to the public without any _____ when the virus of H1N1 hit Mexico in April 2009.

A. delay

B. effort

C. schedule

D. consideration

17. Hiking by oneself can be fun and good for health. It may also be good for _____ building.

A. respect

B. friendship

C. reputation

D. character

18. In our class, when the bell rang and the teacher closed his book, it was a _____ for everyone to stand up.

A. signal

B. chance

C. mark

D. measure

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