can't leave the office now. A lot of work to be done

应该选C remains
work做“工作”讲时是 名词，纵然有a lot of修饰，仍是一个单数概念，相当于much water之类的
“有许多工作要做”可以说成
A lot of work is left to be done.或者
A lot of work remains to be done.

remain一般不用被动态。

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