单项选择题

According to the passage, genetic engineering can be used to ______.

A．make crops hardier
B．kill weeds through cross-breeding
C．make crops resistant to chemical fertilizers
D．improve the yield and quality of most crops
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单项选择题
The word this in Line 4, Para. I refers to ______.
A．the results of recent research
B．dangers inherent in the nature of weeds
C．risks of altering crops' genetic make-ups
D．threats posed by chemicals used to kill weeds
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