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

A

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

B

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

C

$$\frac{51}{56}$$

D

$$\frac{19}{51}$$