Angelica is shopping for a new cell phone plan. she wants a cell phone plan that will cover unlimited calling texting and up to 4gb of data. she finds a plan defined by the function f(x)=9.99x + 120 where x represents the additional gb of data. $9.99 is the cost of the additional data, $120 is the cost of the cell phone plan up to 4gb and c(x) is the total cost. what is the domain of the function?