Xcellenta Systems is a local software development firm that is currently evaluating six different potential projects. There are not sufficient resources to take on all of these, but more than one will be selected. A $0-1$ integer programming model is planned to help determine which of the six opportunities to choose, and variables X$1,$ X$2,$ X$3,$ X$4,$ X$5,$ and X$6$ are being used to represent the six choices. For each of the following situations, please write a constraint $($or several constraints$)$ that would accomplish the stated restriction: