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A. Noise is a major problem; most people recognize its importance.
B. Although noise can be annoying, it is not a major problem.
C. Noise is a major problem and has not yet been recognized as such.
D. Noise is a major problem about which nothing can be done.

A. The number of confirmed cases of swine flu has tripled in the past month.
B. Australia arouses the world’s attention because of the number of its infected cases.
C. Australia accorded its first case of H1N1 on May 9.
D. Many people are infected with H1N1 in its capital Melbourne.