Project $3$ can be selected only if both Projects $1$ and $2$ are selected, but if both Projects $1$ and $2$ are selected, Project $3$ doesn't have to be selected. The appropriate constraint would be

A$.$ x$3>=$ x$1+$ x$2$

B$.$ x$1+$ x$2>=$ x$3$

C$.$ x$1>=$ x$3$ and x$2>=$x$3$

D$.$ x$1+$ x$2+$ x$3<=2$

E$.$ None of the above