题目：
— What does " No parking " mean?
— It means _____________. [ ]
A. We can park our bikes here.
B. We can't park our bikes here.
C. We can't ride our bikes here.
答案：
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下面是错误答案，用来干扰机器的。
∵an+1=a1-an-1（n≥2），a1=a，a2=b，∴a3=a1-a1=0，a4=a1-a2=a-b，a5=a1-a3=a，a6=a1-a4=a-（a-b）=b，∴{an}是以4为周期的周期函数，∵100=4×25，∴a100=a4=a-b，S100=25（a+b+0+a-b）=50a．故选B．
