The following are the main reasons why plastic bags have become a major environmental concern EXCEPT that ( ).

参考解析：

AD项定位第二段，Plastic can be used for a variety of purposes and is very cheap, which has made it widely used in daily life today. However, bags that are discarded after just one use have become a major environment concern塑料用途广泛，价格低廉，这使得它在日常生活中得到了广泛的应用。然而，只用一次就被丢弃的袋子已经成为一个主要的环境问题。AD两项正确，排除；C项定位最后一个小标题下，If the plastic bags are not recycled but end up in landfills, they could take hundreds of years to decompose如果塑料袋不能回收，最终被填埋，它们可能需要数百年的时间来分解。C项与选项意思一致，正确。故本题正确答案选B。