在群落交错区内，物种的数目及一些物种的密度有增大的趋势，这一现象被称为___________。
参考答案：边缘效应
—Did you _____ the Dragon Tower?
—Yes, I _____ it was the tallest steel tower in Asia. [ ]
A. hear, hear about
B. hear about, heard that
C. heard about, hear
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