答案：A

题目分析：考查固定句型。固定句型：祈使句+连词+陈述句=If条件句，主句。本题Turn off your TV or computers when you’re not using them and you’ll greatly reduce it．="If" you turn off your TV or computers when you’re not using them, you will greatly reduce it.故A正确。

点评：本题的句型是高考中非常重要的句式：祈使句+连词+陈述句=If条件句，主句。还可以进一步转化成：名词短语，连词+陈述句。