3.给出以下4个命题①若lim_(x to infty)f(x)=a，则lim_(n to infty)f(n)=a.②若lim_(n to infty)f(n)=a，则lim_(x to infty)f(x)=a.③若lim_(x to x_{0)}f(x)=a，且lim_(n to infty)x_(n)=x_(0)，则lim_(n to infty)f(x_(n))=a.④若lim_(x to infty)x_(n)=x_(0)，且lim_(n to infty)f(x_(n))=a，则lim_(x to x_{0)}f(x)=a.其中真命题个数为（）A. 0.B. 1.C. 2.D. 3.

3.给出以下4个命题 ①若$\lim_{x \to \infty}f(x)=a$，则$\lim_{n \to \infty}f(n)=a$. ②若$\lim_{n \to \infty}f(n)=a$，则$\lim_{x \to \infty}f(x)=a$. ③若$\lim_{x \to x_{0}}f(x)=a$，且$\lim_{n \to \infty}x_{n}=x_{0}$，则$\lim_{n \to \infty}f(x_{n})=a$. ④若$\lim_{x \to \infty}x_{n}=x_{0}$，且$\lim_{n \to \infty}f(x_{n})=a$，则$\lim_{x \to x_{0}}f(x)=a$. 其中真命题个数为（）

A. 0.

B. 1.

C. 2.

D. 3.