A company sells two models of chairs $.-$ a standard model and a deluxe model. The standard model sells for $$100$ and has a variable cost of $$60$ per unit. The deluxe model sells for $$150$ and has a variable cost of $$75$ per unit. Fixed costs are $$180,000$ per month. The sales mix is $60\%$ standard model and $40\%$ deluxe model. What is the contribution margin $($CM$)$ ratio for the company?

a$.60\%$

b$.21\mathrm{.}6\%$

c$.51\mathrm{.}4\%$

d$.40\%$