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
That genetically-modified crops could have harmful effects that can be deduced from the fact that ______.
A．cross-breeding is a natural process
B．hybrids are generally more successful at breeding than natural plants
C．gene movement between cultivated plants and wild ones is inevitable
D．new research shows that genetically-modified plants are likely to develop into weeds
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