Tor nodes question

[mw_prev1@xxxxxxxxxxx]

I've just started using Tor and would appreciate
any help with a few points I'm unsure about.

1. Is it possible to restrict entry/exit nodes by
geographical location of the node?

2. Is it possible to ensure that each node in the
circuit is located in a different country?  What
are the chances that all 3 nodes will be located
in the US, for example?  Is there any way to
tell if this has happened?

3. Is it possible to trace individual packets back
through the 3-node circuit?  If the exit node
knows which node sent the packet, and that node
(middle node) knows the entry node, and the entry
node knows your IP, then couldn't someone trace 
any packets leaving the exit node back to source?

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  mw_prev1@xxxxxxxxxxx

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