The Ravi team has uploaded the exchange on their YouTube (Bravo!):
The above "short" response and my previous upload has the enhanced audio making Ravi's initial response more audible.
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For more clear thinking like this from Ravi Zacharias... I invite you to visit: http://www.rzim.org/
Also, the entire (free) presentation can be found here: http://tinyurl.com/m3gk9hl