国防的主体是国防活动的实行者和主要受益者，通常为（）。
A.国家
B.民族
C.军队
D.政党
参考答案：A
Susan Jones was at the bus stop well on time to take the 7:01 bus, but she had to miss her breakfast to do it.
A．catch up with
B．catch
C．run up to
D．be catching
根据基础高度hb确定可能坠落范围半径R。当（）时，R为5m。
A、2m≤hb≤5m
B、5m＜hb≤15m
C、15m＜hb≤30m
D、hb＞30m
