Answer the following question given this paragraph: 

Check Your Learning Ammonia can be liquefied by compression at room temperature; oxygen cannot be liquefied under these conditions. Why do the two gases exhibit different behavior? Answer: The critical temperature of ammonia is 405.5 K, which is higher than room temperature. The critical temperature of oxygen is below room temperature; thus oxygen cannot be liquefied at room temperature.


Q: Unlike ammonia, oxygen cannot be liquefied at room temperature because its what is below room temperature?


A:
critical temperature