What does "So what then?" mean?

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Jun 5, 2012 01:40 usage English conversation question
I was watching a TV show. Then, I heard this sentence. What does it mean?
When can I use it?

A: Does nobody here speak English?
B: No, he can.
A: Do you speak English? (facing on Mr.C)
C: Yes.
A: So what then? <--------★

I hope you will tell me about the meaning and usage.

Thank you!