单选题 以下代码中"max_value "的输出结果是什么? ```python def find_max_value(lst): if not lst: return None max_value = lst[0] for num in lst: if num > max_value: max_value = num return max_value lst = [7, 9, 11, 15, 9, 7, 18] max_value = find_max_value(lst) print(max_value) ```