题干
Bob promises to join in the football match ________ he has to help his parents on the farm.
A：if
B：as
C：unless
D：when
答案(点此获取答案解析）
C
Bob promises to join in the football match ________ he has to help his parents on the farm.
A：if
B：as
C：unless
D：when
，地面拦截系统同时以初速度v2竖直向上发射一颗炮弹拦截（炮弹运动过程看作竖直上抛），设此时拦截系统与飞机的水平距离为s，若拦截成功，不计空气阻力，则v1、v2的关系应满足（ ）
