If $a, b, c$ are in HP, then what is $\frac{1}{b-a} + \frac{1}{b-c}$ equal to?

1. $\frac{2}{b}$

2. $\frac{1}{a} + \frac{1}{c}$

3. $\frac{1}{2} \left( \frac{1}{a} + \frac{1}{b} + \frac{1}{c} \right)$

Select the correct answer using the code given below:

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