单选题
Only when______ can the postal code system achieve its full efficiency.
A、
all the post offices in the country have the fight machines
B、
all the post offices have trained staff
C、
most post offices have the right machines
D、
most post offices have trained staff
【正确答案】
A
【答案解析】
第三段第二段指出，事实上只有12个邮局有机器在全面工作，而整个系统要在全国普及后才能取得充分效益。
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