题干
—Did you go to the zoo yesterday?
—Yes. The bus trip us about 4 hours. There was too much traffic on the road.
A：took
B：paid
C：spent
D：cost
答案(点此获取答案解析）
A
—Did you go to the zoo yesterday?
—Yes. The bus trip us about 4 hours. There was too much traffic on the road.
A：took
B：paid
C：spent
D：cost
}中, 
是其前n项和，
，则
=（）
