下列函数可作为连续型随机变量 $X$ 的概率密度的是
A
$f(x)=\left\{\begin{array}{l}2,-1 < x < 1, \\ 0, \quad \text { 其他．}\end{array}\right.$
B
$f(x)=\left\{\begin{array}{lc}x^2, & -1 < x < 1, \\ 0, & \text { 其他．}\end{array}\right.$
C
$f(x)=\left\{\begin{array}{lc}\frac{1}{2}, & -1 < x < 1, \\ 0, & \text { 其他．}\end{array}\right.$
D
$f(x)=\left\{\begin{array}{l}x,-1 < x < 1, \\ 0, \quad \text { 其他．}\end{array}\right.$
E
F