Suppose you are building a linear optimization model to help decide where to build new cell towers for your mobile phone company. There are three possible locations for your cell towers which are referred to as site A$,$ site B $,$ and site C$.$ The decision variables are $,$ and Let XA $=1$ if you choose to build a cell tower on site A$,$ and if you choose NOT to build a cell tower on site A$.$ Let xB $=1$ if you choose to build a cell tower on site B$,$ and if you choose NOT to build a cell tower on site B$.$ Let xC $=1$ if you choose to build a cell tower on site C and if you choose NOT to build a cell tower on site C$.$ Because of the geography of the locations and your desire to provide adequate cell coverage for customers, you know that if you choose to build a cell tower on site Athen you MUST also build cell towers on site locations B and C$.$ If you choose not to build a cell tower on site location A$,$ then there is no requirement on whether or not you build a cell tower on site locations B and C$.$ Which of the following constraints will achieve this logical constraint? xA $<=$ xB $-$ xC; xA $<=$ xB $+$ xC; XA $+$ XB $+$ XC $=1$ Requires $2$ constraints: $<=$xB AND XA$<=$xC