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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假设你是张华，请给你的一位朋友写封信，介绍你以前所用过的一本好的参考书，并建议他如何使用该书。An Introduction to a Friend of a Good Reference Book

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