$X \sim N(0,4)$, 则 $P(X < 1)=(\quad)$ 。
A. $\int_0^1 \frac{1}{2 \sqrt{2 \pi}} e^{-\frac{x^2}{8}} d x$
B. $\int_0^1 \frac{1}{4} e^{-\frac{x^2}{4}} d x$
C. $\frac{1}{\sqrt{2 \pi}} e^{-\frac{1}{2}}$
D. $\int_{-\infty}^{\frac{1}{2}} \frac{1}{\sqrt{2 \pi}} e^{-\frac{x^2}{2}} d x$