单选题
Which question is NOT answered in the article?
A、
How many plant species found outside tropical rain forests could be used in the treatment of cancer?
B、
Which plant has a product used for tile treatment of anxiety?
C、
Worldwide, what is the total annual production of synthetic rubber?
D、
Which rain forest plant can be used for the treatment of eye disease?
【正确答案】
C
【答案解析】
文中并未说明人工合成橡胶的年总产量。
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