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单选题
When the smoke control relay R648 energizes, it supplies 28v dc to energize the supply system control interrupt relay R645.
A
When the smoke control relay R648 is powered, it gives 28v dc to activate the supply system control interrupt relay R645 .
B
As the smoke control relay R648 is connected, 28v dc is supplied to the supply system control interrupt relay R645.
C
When the smoke control relay R648 has power, it needs 28v dc to start the supply system control interrupt relay R645.
答案解析
正确答案：A
