A well in the uppermost deck of a shelter deck vessel which has only a temporary means of closing for the purpose of gaining an exemption from tonnage measurement is called a（n）（）.

A.Exemption space

B.Tonnage deck

C.Cofferdam

D.Tonnage opening

癌胚抗原是肿瘤细胞所特有的新抗原，它只表达于肿瘤细胞，而不存在于正常组织细胞。