单选题 13. Growing grapes in greenhouses containing air enriched with carbon dioxide increases crop yields, an effect that is enhanced when the growing temperature is also raised several degrees. Although enclosed cultivation is impractical in vineyards, increases in yields from grape cultivation in the open air can be expected because emissions from the burning of fossil fuels will almost certainly lead to an increase in the carbon dioxide content of atmospheric air over the next century.
Which of the following, if true, most weakens the argument? ______

A、 When grapes are grown in greenhouses, it is relatively easy to control the grape pests that thrive in air enriched with carbon dioxide.
B、 Recent improvements in pest control have increased grape yields in many vineyards.
C、 Increasing carbon dioxide levels in greenhouses has not been shown to improve the quality of grapes grown in greenhouses.
D、 Increasing carbon dioxide levels in greenhouses increases the sweetness of raisins made from grapes grown in the greenhouses.
E、 As the carbon dioxide level in the open air increases, temperatures at the Earth's surface will also increase.

【正确答案】 A

【答案解析】 A：二氧化碳上升时，在露天环境下控制害虫不容易→产量下降。削弱了结论，故正确。
B：该选项加强了结论，故不选。
C：文章讨论的是产量，和品质无关，故不选。
D：错误原因同选项C。
E：温度和二氧化碳都上升，根据文章可知这会导致葡萄产量上升。加强了结论，故不选。