The half life of a radioactive substance is 20 minutes. The approximate time interval $$(t_2 - t_1)$$ between the time $$t_2$$ when $$\dfrac{2}{3}$$ of it has decayed and time $$t_1$$ and $$\dfrac{1}{3}$$ of it had decayed is:

A

$$14 \, \text{min}$$

B

$$20 \, \text{min}$$

C

$$28 \, \text{min}$$

D

$$7 \, \text{min}$$