下列模型中属于人口模型的是（）A. } (dx)/(dt) = rx^2 + C x(0) = x_0 B. m (dv)/(dt) = mg - rho g V - kv^2C. int (dx_1)/(dt) = r_1 (1 - (x_1 + alpha_1 x_2)/(K_1)) x_1

A. $\begin{cases} \frac{dx}{dt} = rx^2 + C \\ x(0) = x_0 \end{cases}$

B. $m \frac{dv}{dt} = mg - \rho g V - kv^2$

C. $\int \frac{dx_1}{dt} = r_1 \left(1 - \frac{x_1 + \alpha_1 x_2}{K_1}\right) x_1$