执行以下python程序后所输出的结果是?( )
def f(n): s=0 for i in range(2, n+1): p = True for j in range(2, int(i**0.5)+1): if i % j == 0: p = False break if p==True and i%3==1: s=s+i return s print(f(13))
20
41
21
91