28. Please answer to the questions by the following article：To ensure the effective implementation of the provisions of the ship security plan, drills should be conducted at least once every three months, in cases where more than 25 percent of ship's personnel that has not previously participated in any drill on the ship, within the last months, a drill should be conducted within one week of the change. These drills should test individual elements of the plan. If there are twenty crewmembers on your ship and three members have been changed at this port, the ship took place the drills last month, you should do ISPS drills within _____. (A) six months (B) one week (C) twenty-four hours (D) keep going as usual