单项选择题

A．It breaks down at times.
B．The alarms sometimes do not sound.
C．There are a lot of false alarms for small quakes.
D．The computer controlling the system gives wrong orders occasionally.
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单项选择题
A．They have been working at the telescope for two days.B．They can’t find the microscope.C．They’ve been working in the laboratory for two hours.D．They can’t fix the microphone.
A．They have been working at the telescope for two days.
B．They can’t find the microscope.
C．They’ve been working in the laboratory for two hours.
D．They can’t fix the microphone.
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