The Cost Function for a Certain Graphing Calculator Is Given $C = 11.35 x + 53,000$

The cost function for a certain graphing calculator is given by $C = 11.35 x + 53,000$ where $C$ is in dollars and $x$ is the number of calculators produced.
a. Write a model for the average cost per unit produced.
b. Find the average cost per unit when $x = 900$ .
c. Determine the limit of the average cost function as $x$ approaches $\infty$ .