Question to Powell: Why is the US dollar the world's reserve currency?

A question to Powell has come in towards the end of the Q&A session at the two-day Congressional testimony and was asked, 'why is the US dollar the world's reserve currency?".

Powell answered as follows:

• There tends to be one reserve currency and there tends to be a stable equilibrium.
• It's very long run thing.
• What could compete with the dollar, the euro? - It is hard to see that the dollar will not be the reserve currency for quite some time. 
• I don't see it under threat.
• In the long run, rising debt would threaten the dollar's status.