A:can have used
B:need have used
C:must have used
D:could have used
— _______ does he usually go to work?
— He walks to work.
植物种类
植被覆盖率
主要的植物
改造前
20种
9%
蒲公英、狗尾草、艾蒿
改造后
33种
27%
芦苇、菖蒲、睡莲