if $\mathrm{n}\left(\mathrm{A}\right)=3, \mathrm{n}\left(\mathrm{B}\right)=6 \mathrm{and} \mathrm{A}\subseteq \mathrm{B}.$ Then the number of elements in $\mathrm{A}\cup \mathrm{B}$ is equal to

• A

  3

• B

  9

• C

  6

• D

  8