All the gifts must be mailed immediately( )in time for chris

so as to be received
so as to = in order to 为了、便于
后面跟动词原型,其实这个动词与to一起构成动词不定式,动词不定式视语境还要作各种时态及语态上的变化
本句：所有礼物必须立即寄走,（以便于）这些礼物可以在圣诞节准时（被收到） 用动词不定式的被动语态 to be received
所以只有so as to be received正确