Use the following information about sales and costs to prepare a scatter diagram. Draw a cost line that reflects the behavior displayed by this cost. Determine whether the cost is variable, step$-$wise, fixed, mixed, or curvilinear.

$\backslash$table$[[$Period$,$Sales,Costs,Period,Sales,Costs$],[1$dotsdotsdotsdots,$$760,$$$590,9$dotsdotsdotsdotsdots,$$580,$$$390$