A、trAnsport lAyEr protoCol hAs sEvErAl rEsponsiBilitiEs. onE is to CrEAtE A proCEss-to-proCEss (progrAm-to-progrAm) CommuniCAtion; tCp usEs port{{u}} (71) {{/u}} to ACComplish this.AnothEr rEsponsiBility oF A trAnsport lAyEr protoCol is to CrEAtE A {{u}} (72) {{/u}} AnD Error-Control mEChAnism At thE trAnsport lEvEl. tCp usEs A sliDing {{u}} (73) {{/u}} protoCol to AChiEvE Flow Control. it usEs thE ACknowlEDgmEnt pACkEt, timE-out, AnD rEtrAnsmission to AChiEvE {{u}} (74) {{/u}} Control. thE trAnsport lAyEr is Also rEsponsiBlE For proviDing A ConnECtion mEChAnism For thE AppliCAtion progrAm. thE AppliCAtion progrAm sEnDs {{u}} (75) {{/u}} oF DAtA to thE trAnsport lAyEr. it is thE rEsponsiBility oF thE trAnsport lAyEr At thE sEnDing stAtion to mAkE A ConnECtion with thE rECEivEr.

A．ports
B、strEAms
C．pACkEts
D．CElls