强心苷的负性频率作用可表现为（）

A.心电图P-R间期缩短

B.心电图Q-T间期缩短

C.心电图Q-T间期延长

D.心电图P-R间期延长

E.心电图P-P间期延长

The Urgency Signal（）indicates that the station calling has a very urgent message to transmit concerning the safety of a ship or aircraft，or of some person on board or within sight.

A.SOS

B.XXX

C.TTT

D.PAN