我有點陷入這個挑戰目標是：“創建一個函數來計算句子中有多少個 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這是我當前的代碼：def count_d(sentence):    print(sentence)    sentence = sentence.lower    substring = "d"    return sentence.count(substring)當我運行它時，控制臺發送一條錯誤消息：ERROR: Traceback:   in <module>   in count_dAttributeError: 'builtin_function_or_method' object has no attribute 'count'