Each year in the United States, many black teenagers _______ of school, either because they cannot keep up or they have to work to support their family.

________ the concert had begun did we realize what a splendid show it was going to be.

2018N18A DNA 在融解时温度的变化是？

新民主主义社会的基本阶级力量主要有___________。

“德里苏丹国”名称解释