以下哪个函数可以成为某个随机变量的分布函数
A. $F (x)=\left\{\begin{array}{cc}0, & x \leq 2, \\ 0.4, & 2 < x < 3, \\ 1, & x \geq 3 .\end{array}\right.$
B. $F (x)=\left\{\begin{array}{cc}0, & x < 0, \\ \frac{x^2}{4}, & 0 \leq x < 2, \\ 1, & x \geq 2 .\end{array}\right.$
C. $F (x)=\left\{\begin{array}{cc}0.2, & x=0, \\ 0.6, & x=1, \\ 0, & \text { 其它．}\end{array}\right.$
D. $F (x)=\left\{\begin{array}{cc}2-e^{-\frac{x}{2}}, & x>0, \\ 0, & x \leq 0 .\end{array}\right.$