—Must I let my mother know it?—.You can keep it as a secret.A. Yes, you must B. No, you mu

—Must I let my mother know it?
— .You can keep it as a secret.
A. Yes, you must B. No, you mustn’t C. No, you needn’t

解析： Must I…?肯定回答Yes, you must，否定回答No, you needn’t。根据句意：我必须让妈妈知道吗？-----，你可以把它当作一个秘密，故选C。

对的，就是这个意思