Which of the following is a null set?

 

• A

  $\left\{0\right\}$

• B

  $\{x:x>0\text{ or }x<0\}$

• C

  $\left\{x:x^2=4\text{ or }x=3\right\}$

• D

  $\left\{x:x^2+1=0,x\in R\right\}$