A：$\exists x\in R,f\left(x\right)\le f\left(x_0\right)$

B：$\exists x\in R,f\left(x\right)\ge f\left(x_0\right)$

C：$\forall x\in R,f\left(x\right)\le f\left(x_0\right)$

D：$\forall x\in R,f\left(x\right)\ge f\left(x_0\right)$