若|x|=﹣x,则x是[ ]
A.正数
B.负数
C.负数或零
D.正数或零
答案:C
While watching television, __________.
A.the doorbell rang
B.the doorbell rings
C.they heard the doorbell rings
D.they heard the doorbell ring
在一次回风空调系统中,改变机器露点的调节方法有( )种。
A.3 B.4 C.5 D.6