The middle term in the expansion of $$\left(x^2 - \frac{1}{2x}\right)^{10}$$ is

A

$$\frac{56}{7}x^5$$

B

$$-\frac{36}{7}x^5$$

C

$$-\frac{63}{8}x^5$$

D

$$-\frac{72}{5}x^5$$