EXERCISES $33$ a$)$ Use the given information to complete the model. That is$,$ find the transmission and attrition coe ffi cients a and b$.$ b$)$ Using At $=2,$ estimate S $(4),1(4),$ and R $(4).$ c$)$ Find the threshold and explain its significance. d$)$ Suppose that upon becoming active, customers are given personal customer support and continual product updates that extend the time they stay active to an average of $30$ days. What e ect will this have on the threshold for S$?$ Do you think the improvement in the threshold is likely to be worth the extra cost of the intervention?