Suppose Xi $($i $=1,2)$ is $1$ if project i is selected and $0$ if project i is not selected. To ensure that project $1$ cannot be selected unless project $2$ is also selected, one would include which of the following constraints?

L$$tfen birini se$$in:

a$.$X$1+$ X$2=1$

b$.$X$1+$ X$2<=1$

c$.$ None of them.

d$.$X$1-$X$2<=0$

e$.$X$1-$X$2=1$