Simplify the expression $$\frac{21+\left(-18\right) \times 2\left[-4 + 15 \left\{5\div \left(4-7\right)\right\} \div 4 \left(-4\right)\right]}{\left(-14\right) \times \left(2\right) + 7}$$

A

$$\frac{7}{20}$$

B

$$-\frac{5}{12}$$

C

$$\frac{7}{12}$$

D

$$3\frac{8}{21}$$