Question 178929
 A tree has green leaves or the tree does not produce oxygen. 
     This tree has green leaves 
     ------------------------------------------------------------------------ 
     \ This tree does not produce oxygen. 
:
<pre>
p=tree has green leaves. q=tree does not produce oxygen 
p   q   pvq 
t   t    t      tree has green leaves. tree does not produce oxygen 
t   f    t       tree has green leaves. tree produces oxygen 
f   t    t       
f   f    f
:
 since it is an "or" this argument is invalid       
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