热煤气站至最远用户之间热煤气管道的长度,应根据煤气在管道内的温度降和压力降确定,但不宜超过()m。
A、50
B、80
C、100
参考答案:B
(2013·湖南,35)Not once________to Michael that he could one day become a top student in his class.
A.occurred it
B.it did occur
C.it occurred
D.did it occur
20℃时将a克氯化钠加入b克水中,充分溶解后得溶液c克,则a、b、c的关系表示正确的是[ ]
A.a+b=c
B.a+b≤c
C.a+b≥c
D.a+b>c