Let XA and XB be decision variables corresponding to taking up project A and project B $($eg: XA $=0$ if the decision is donot take up project A and XA $=1$ if take up project A; similarly XB $=0$ if decision is don$$t take up project B and XB $=1$ if take up project B$).$ How to model the following constraints? $($write out an inequality or equation that conveys the logic below$)($each worth $1$ point$)$: $1.$ Can undertake project A or B but not both $2.$ Can undertake project A only if project B is also done $3.$ Can undertake project A and project B only if done together $4.$ Have to undertake project A if project B is taken up for investment