If the machine makes a mistake -- rarely happens -- it gives a warning in the form of a red light.
A. that
B. which
C. as
D. what
网友回答
A. that
(答案→)B. which
C. as
D. what
解析：考查定语从句。本定语从句中which指代一句话的内容，在非限制性定语从句中做主语。That不能引导非限制性定语从句，as有正如的意思，what不是定语从句的引导词。句意：如果这个机器犯了错误，这很少发生，它就会以红色的灯光发出警报。故B正确。