Which of the following formula is used to calculate the total resistance R of the circuit shown below?

A

$$ R = \left( \frac{1}{R1} \right) + \left( \frac{1}{R2} \right) + .... + \left( \frac{1}{Rn} \right)$$

B

$$ \frac {1} {R} = \left( \frac{1}{R1} \right) + \left( \frac{1}{R2} \right) + .... + \left( \frac{1}{Rn} \right) $$

C

$$ \frac {1} {R} = R1 + R2 +... + Rn $$

D

$$ R = R1 + R2 +....+Rn $$