运行以下程序，是分析其输出结果为（ ）。def flat(s): lst=[] for t in s: if isinstance(t,list): for c in flat(t): lst.append(c) else: lst.append(t) return lstdef main(): ls=[2,3,[4,[5,6],7],[8,9,10]] fls=flat(ls) print(fls[3])if __name__=="__main__": main()

A. 4
B. 5
C. 8
D. [8,9,10]