A:水蒸气
B:二氧化碳气体
C:洁净的空气
D:氯酸钾
医院里的B超机是利用超声波来诊断病情的,但是人们却听不到它发出的声音,这是因为 ( )
____Six.
____I have computer class, PE and science.
____How many classes do you have on Wednesdays?
____Is it Wednesday today?
____What do you have in the afternoon?
____Yes, it is.