Why do airliners fly at 35,000 feet?

Why do airliners fly at 35,000 feet?
Very, VERY late response, but the reason that planes fly at 35k mainly has to do with the engines. The conditions at that elevation are stable in regards to temperature and pressure, and are the most optimal and fuel efficient conditions for turbines.
 
If they flew lower, they'd hit mountains, birds, power lines and trees?...
 
They tried flying at 35,001 but the pilots complained that trying to keep that one foot was too taxing. The FAA and the company offered 34,999, the union countered with 35 even and the rest is history.