Here are the decision variables and constraints of an integer linear optimization model: x$1,$ x$2$ are integer x$1$ x$2>=1\mathrm{.}5$ x$1>=1$ x$1<=3$ x$1,$ x$2>=01.$ How many feasible decision points are there? $2.$ If the objective function is to maximize x$1,$ then how many optimal decision points are there? $3.$ If the objective function is to maximize x$1+$ x$2,$ then what is the optimal objective value?