This is the 2306th most frequent Chinese word.
难道
"难道" is used to introduce a rhetorical question, meaning "Could it be that...?" or "Is it possible that...?"
In this sentence, "难道" introduces a rhetorical question that expresses incredulity or surprise.
难道这不是你的书吗？
Isn't this your book?
Here, "难道" emphasises the speaker's disbelief regarding the listener's attitude towards their friend.
难道你不关心你的朋友吗？
Don't you care about your friend?
Using "难道" conveys the speaker's astonishment and implied disagreement with the proposed resolution.