根据材料内容, 回答下列问题。
    Some boys like wearing the same kinds of clothes as girls, and some of them have long hair, so sometimes it's difficult to tell whether (是否)they are boys or girls.
    Today is Sunday. I have a walk with my grandmother in the park. My grandmother is 80 years old and she can't see well. After some time, my grandmother is tired (疲倦的), so she sits down on a chair. There is another person sitting on the chair, too.
   “Hello,” my grandmother says to the person with long hair next to her. “Can you see the person with long hair under the big tree? Is it a boy or a girl? ”
  “Which (哪一个)one? ” asks the person.
  “The one eating under the big tree,” says my grandmother.
  “The boy?” says the person. “He's my brother Mark.”
  “Oh,” says my grandmother, “I don't know you are his sister.”
  “No, I'm not his sister. I'm his brother.”