甲、乙、丙、丁四位志愿者安排在周一至周日的7天中参加今年的上海世博志愿者活动，要求每人参加一天且每天至多安排一人,并要求甲排在乙的前面，则不同的方法有

A．280

B．420

C．840

D．920

The great man who led the people through the was elected president________.     [ ]

A. hard time; the second time      

B. hardly time; for a second time      

C. hard times; for the second time      

D. hard times ; the second time