18．当心脏房室瓣处于关闭而动脉瓣处于开放状态，心房和心室舒缩状况应是

       A．心房心室同时收缩                                    B．心房舒张，心室收缩

       C．心房心室同时舒张                                    D．心房收缩，心室舒张