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阅读理解Cotton bags have become a means for brands, retailers, and supermarkets to promote a planet-friendly concept-or, at least, to show that the companies are aware of the overuse of plastic in packaging. There's a trend in New York right now where people are carrying cotton bags from local bakeries, hardware stores, or their favorite steakhouses, said designer Rachel Comey. So far, so earth-friendly? Not exactly. It turns out the wholehearted embrace of cotton bags may actually have created a new problem. An organic cotton bag needs to be used 20,000 times to offset (抵消) its overal已知向量夹角为45°，且，则=    ．