3. $0<a \leqslant \frac{1}{5}$ is the ( ) for the function $f(x)=a x^{2}+2(a-1) x+2$ to be a decreasing function on the interval $(-\infty, 4]$.
A. Sufficient but not necessary condition
B. Necessary but not sufficient condition
C. Sufficient and necessary condition
D. Neither sufficient nor necessary condition
3. A. $f(x)$ is a decreasing function on $(-\infty, 4)$ $\Leftrightarrow 0 \leqslant a \leqslant \frac{1}{5}$.