A.1:2
B.2:1
C.1:4
D.4:1
答案:D
—How often do you walk to school?
—______. [ ]
A. Never
B. Two hours
C. 1 point
D. by bus
节流膨胀过程是使介质()。
A、制冷
B、降温
C、作功
D、都不是