Editor
The (a)M between $a_{n-1}$ and $a_{n+1}$ is:
Difficulty: Easy
A: 
a+b/2
B: 
$a_{n+1}-a_{n-1}$
C: 
$\frac{a_{n+1}-a_{n-1}}{2}$
D: 
$\frac{a_{n-1}-a_{n+1}}{2}$
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