管道比阻
参考答案：
每一米或每千米管道的阻力。
The surroundings of his house _____ clean now. [ ]
a. is
b. are
c. was
d. were
甲企业废水处理前氨氮年排放量为（）t。
A.12
B.120
C.1200
D.1.2
