The sum of the order and degree of the differential equation $\left[1+\left(\frac{\text{d}y}{\text{d}x}\right)^2\right]^3 = \frac{\text{d}^2y}{\text{d}x^2}$ is : (A) $2$ (B) $\frac{5}{2}$ (C) $3$ (D) $4$