质点作半径为R的变速圆周运动时的加速度大小为(v表示任一时刻质点的速率)
$\text{A.}$ $\frac{d t}{d t}$.
$\text{B.}$ $\frac{v^2}{R}$.
$\text{C.}$ $\frac{\mathrm{d} v}{\mathrm{~d} t}+\frac{v^2}{R}$.
$\text{D.}$ $\left[\left(\frac{d y}{d t}\right)^2+\left(\frac{y^4}{R^2}\right)\right]^{1 / 2}$