阅读---给建议栏目请求帮助。

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What should I do?
3a.Read and find out the problem.
Dear Mary,
    I have a problem, and I need your help. I
always thought I was popular at school. But
I just found out that my friends were
planning a birthday party for my best friend,
and they didn’t invite me. Everyone else in
my class was invited except me, and I don’t
know why. I can’t think what I did wrong.
I’m very upset and don’t know what to do.
What do you think? Can you help me?