皮质层
参考答案：
约占80%头发的主要组成部分，由角蛋白组成成纤维状。含有大量的自然色素粒子，皮质层反应出头发的颜色，染发、烫发均是在这一层起作用。
句型转换。
1. He is writing a letter to his mother. (变一般疑问句)
_________ he _________ a letter to his mother?
2. We went by bus. (对画线提问)
_________ did _________ go?
3. I can send my friend an e-mail. (同义句)
I can send an e-mail _________ _________ _________.
4. You were sleeping on the train. (改为一般疑问句)
_________ _________ sleeping on the train?
5. He has a computer. (否定句)
He _________ _________ a computer.
顶板下沉主要发生在（）阶段。
A、开挖过程中
B、上断面开挖
C、下断面开挖及立柱托换
D、支撑拆除
E、施工时
