Mr. Green works in office but his sister works a farm.Aa in B．an in C．an on

问题补充：

Mr. Green works in office, but his sister works a farm.

A．a, in B．an, in C．an, on

答案：

解析：in在里面；on在上面。句意：格林先生在一个办公室工作，但是他的妹妹在农场上工作。Office词首音素是元音，故不定冠词用an，故选C。