were
have been
had been
will be
本题考查过去完成时。
题干中went to the beach 是一般过去时,而busy with our work是发生在went to the beach 之前的事,因此是过去的过去,所以要用过去完成时。故C选项符合题意。
句意:——Peter,你们暑假去哪儿了?——我们忙了好几个月的工作,所以我们就去海滩放松了一下。