字谜

Definition: “ A distinct, nearly level surface of the valley ______. The term first bottom is usually used to mean the flood plain, the lowest, most frequently flooded surface on the valley ______. The terms second ______, third ______... are used to describe terraces above the normal flood plain.”

第一个字母: B

请回答  

这个单词是什么?

"智商" 0
 
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