单项选择题

Enamel, the hardest substance in a human being's body, covers the entire crown of the tooth.

A. whole
B. inseparable
C. superficial
D. main
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Grouping stars by constellations is a handy way of mapping the sky.
A. nice
B. funny
C. manual
D. convenient
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Genetics is the study of the biological factors that influence heredity and determine variation in living things.
A. duplicate
B. affect
C. clarify
D. improve
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