依据CRH1A（时速250公里）型动车组二级（专项）修检修计划，外门检查专项修的维护周期为（）天。
A.30
B.90
C.180
D.360
参考答案：D
They had a good time _______ their school trip. [ ]
A. on
B. in
C. at
D. for
