Avg of $$1$$ \& $$5$$ is $$3$$. Avg of $$1$$ and $$7$$ is $$4$$. Both are whole numbers. 
($$1999$$ Math League, Grade $$5$$, Question \#$$16$$)  The average of two odd numbers is always ． 

(1) odd$$ $$ 
(2) even$$ $$ 
(3) prime$$ $$ 
(4) whole$$ $$ 