In a binary integer programming problem, which of the following constraints will enforce a relationship between project $1$ and $2$ such that project $1$ can be accepted only if project $2$ is also accepted and that project $2$ can only be accepted is project $1$ is accepted?

Select one:

a$.$

x$1<=$ x$2$

b$.$

x$2=$ x$1$

c$.$

x$1+$ x$2<=1$

d$.$

x$1+$ x$2=1$

e$.$

None of the answer choices is correct.