$10\mathrm{.}8.$ Yahoo Finance reported the following market betas for the stocks of selected health insurers:

Company

Beta

UnitedHealth Group $0\mathrm{.}70$

Humana $,1\mathrm{.}16$

Aetna $,0\mathrm{.}63$

Cigna $,0\mathrm{.}71$

Assume that the risk$-$free rate, RF$,$ and required rate of return on the market, $R(RM),$ are $2\mathrm{.}0$ percent and $8\mathrm{.}5$ percent, respectively.

a$.$ What are the required rates of return on the four stocks?

b$.$ Which of the stocks is riskiest for investors?