后生皮层
参考答案：
有的双子叶植物的根，由皮层的外部细胞木栓化起保护作用。
—What is your brother?
—He is _____. [ ]
A. a English teacher
B. a policeman
C. active
D. handsome
They poured waste water the river. [ ]
A. into
B. in
C. to
D. at
