下列二次型中，正定二次型是
A
$f\left(x_1, x_2, x_3, x_4\right)=\left(x_1-x_2\right)^2+\left(x_2-x_3\right)^2+\left(x_3-x_4\right)^2+\left(x_4-x_1\right)^2$
B
$f\left(x_1, x_2, x_3, x_4\right)=\left(x_1-x_2\right)^2+\left(x_2+x_3\right)^2+\left(x_3+x_4\right)^2+\left(x_4+x_1\right)^2$
C
$f\left(x_1, x_2, x_3, x_4\right)=\left(x_1-x_2\right)^2+\left(x_2+x_3\right)^2+\left(x_3-x_4\right)^2+\left(x_4+x_1\right)^2$
D
$f\left(x_1, x_2, x_3, x_4\right)=\left(x_1+x_2\right)^2+\left(x_2+x_3\right)^2+\left(x_3+x_4\right)^2+\left(x_4+x_1\right)^2$
E
F