Several merchant ships are arriving at the scene of a distress incident. One of the them must assume the duties of the Coordinator Surface Search (CSS). Which of the following statements is TRUE?（）.
A.CSS duties are always assumed by passenger vessels，dry cargo vessels，or tankers in that order of precedence
B.The CSS must be established by mutual agreement between the ships concerned
C.A tanker should never be assigned CSS duties unless only tankers are present
D.The first vessel to arrive at the distress incident is designated as the CSS