If $$a_1, a_2, a_3$$ and $$a_4$$ in the increasing order of the elements in the set $$\left\{\frac{2}{7}, \frac{7}{15}, \frac{5}{8}, \frac{9}{23}\right\}$$ then $$a_4 - a_1 = $$

A

$$\frac{2}{7}$$

B

$$\frac{19}{56}$$

C

$$\frac{17}{56}$$

D

$$\frac{15}{56}$$