Everyone had a form in his hand, but no one knew which office____．
A.to send it toB.to sent itC.to be sent toD.to have it sent在线课程A
解析：
考查定语从句。句意：没有人知道把它寄到哪个办公室。句子可还原：不是定语从句时应该是to send it to that office room，要换成定语从句office room 作为先行词提前，并把that 换成 which修饰office。