31. Please answer to the questions by the following article：To ensure the effectiveimplementation of the provisions of the ship security plan, drills should be conducted at least onceevery three months, in cases where more than 25 percent of ship's personnel that has notpreviously participated in any drill on the ship, within the last months, a drill should be conductedwithin one week of the change. These drills should test individual elements of the plan. If there aretwenty crewmembers on your ship and three members have been changed at this port, the shiptook place the drills last month, you should do ISPS drills within _____.
(A) three months
(B) one week
(C) twenty-four hours
(D) keep going as usual