if 虚拟语气有三种情况
对未来虚拟 if sb/sth +should do/were to do/did,sb/sth would do
对现在虚拟 if sb/sth+did,sb/sth would do
对过去虚拟 if sb/sth+had done,sb/sth would have done.
所以本题应看成对未来或者是现在的虚拟,故 用did so 为好