单项选择题
A.90° B.180° C.270° D.150°
A.8V B.18V C.24V D.36V
A.7805 B.7915 C.7812 D.7912
A.低位由0变1时 B.上一级由1变0时 C.下一级由0变1时 D.无规律
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