以下哪个程序段是使用递归函数实现1到100求和 A: def product1(num): product=1 for i in range(1,num+1): product=product *i return productprint(product1(10)) B: def sum1(num): sum2=0 for i in range(1,num+1): sum2+=i return sum2 print(sum1(100)) C: def product2(num): if num==1: return 1 else: return num *product2(num-1)print(product2(10)) D: def sum_a(num1): if num1==1: return 1 else: return num1+sum_a(num1-1) print(sum_a(100))