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$.x_1+x_2=1$

b$.$ None of the answer choices is correct.

c$.x_2=x_1$

d$.x_1{x}_2$

e$.x_1+x_{2}1$