used to give emphasis to a rhetorical question

• 难道你不知道吗？

  Don't tell me you don't know?

• 难道这是偶然的吗？

  So I take it that this happened by chance?

• 难道就这样罢了不成？

  So that's the end of it?

• 难道要我向你请示不成！

  So I need to ask for your instructions?

• 难道不是这样么？

  Is this not the case?

• 他们做得到，难道我们就做不到吗？

  if they can do it, why can't we?