Two known resistances of $$R \Omega$$ and $$2R \Omega$$ and one unknown resistance $$X \Omega$$ are connected in a circuit as shown in the figure. If the equivalent resistance between points A and B in the circuit is $$X \Omega$$, then the value of X is __________ $$\Omega$$.

A

$$2\left(\sqrt{3}-1\right)R$$

B

$$\left(\sqrt{3}-1\right)R$$

C

R

D

$$\left(\sqrt{3}+1\right)R$$