人体不能消化的物质是 [ ]
A．维生素C
B．蛋白质
C．淀粉
D．纤维素
答案：D
The boys went home by bus _____ the end of the day. [ ]
A. on
B. in
C. at
D. by
什么是可燃性混合物的爆炸极限，其受哪些因素影响？
