【题目】All these postcards must be mailed at once in time for the Spring Festival.
A.to have received
B.so as to be receiving
C.so as to be received
D.to receive

【答案】C
【解析】考查非谓语动词。句意：所有这些贺卡必须立即寄出去，是为了使它们在春节及时被收到． “receive”和“postcards t”之间是被动关系，所以so as to be received正确。故选C。