我有点陷入这个挑战目标是：“创建一个函数来计算句子中有多少个 D”。一些例子：count_d("My friend Dylan got distracted in school.") ➞ 4count_d("Debris was scattered all over the yard.") ➞ 3count_d("The rodents hibernated in their den.") ➞ 3这是我当前的代码：def count_d(sentence):    print(sentence)    sentence = sentence.lower    substring = "d"    return sentence.count(substring)当我运行它时，控制台发送一条错误消息：ERROR: Traceback:   in <module>   in count_dAttributeError: 'builtin_function_or_method' object has no attribute 'count'