VOA慢速英语2012.07.20
[audio]http://sound.yywz123.com/tingbbs/voa/voa20120720.mp3[/audio]When is a plant considered a weed? Experts at Penn State University have a simple answer: When the undesirable qualities outweigh the good qualities. Consider the fact that crops generally produce several hundred seeds from each plant. 植物什么时候会被当成杂草？宾夕法尼亚州立大学的专家们给出了一个简单的答案：当其不良品质超过优良品质时。我们要考虑到每株农作物通常会产出几百颗种子。
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