问题：

[单选题]In the video lecture we see a comment in the code: "Two base cases are required". What does it refer to? 视频里代码注释中“需要两个递归基”的含义是P9D答案窝(daanwo.com)-大学作业答案分享平台
AWe need two functions handling different cases (when n is even or odd). 问题需要按照“n为奇数”、“n为偶数”两种情况分别设计两个函数P9D答案窝(daanwo.com)-大学作业答案分享平台
BThe sequence of recursive callls is terminated when the problem size is reduced to either 0 or 1. 在问题规模缩减为0或1时，停止递归P9D答案窝(daanwo.com)-大学作业答案分享平台
CThe program returns to the main function when the problem size is reduced to either 0 or 1. 在问题规模缩减为0或1时，返回main函数（或递归函数被调用的函数）P9D答案窝(daanwo.com)-大学作业答案分享平台
DTwo sub-instances are generated from every instance of the recursive calls. 递归函数在执行过程中将每次创建两个递归实例