As a project manager, Mary is managing $12$ different projects. She realized something very interesting related to them. Sometimes they present some unique and particular rules, like the ones for projects #: $3,6,$ and $8.$ She concluded that if either project $3$ or project $6$ is selected, then project $8$ must be selected.

For integer programming problems, which one of the alternatives listed below correctly models this situation?

$x3+x6+x81$

$x3-x81,x6-x81$

$x3+x81,x6+x81$

$x6x8,x3x8.$

None of the above