题目

They leap from helicopters or speeding boats, bringing aid to swimmers who get into trouble off Italy’s popular beaches. For these dog lifeguards, the doggie paddle （狗刨式游法）does just fine. These “lifedogs” tow a buoy(救生衣) that victims can grab, or a raft they can sit on to be towed back to shore, and unlike their human counterparts(与对方地位相当的人), they can easily jump from helicopters and speeding boats to reach swimmers in trouble. With millions flocking(人群) to Italy’s crowded beaches each summer, the Italian Coast Guard says it rescues about 3,0008名学生站成两排，前排3人，后排5人，则不同站法的种数为 ①+； ②； ③+； ④.其中正确命题的个数是(      )A．0B．1C．2D．3