The market risk, beta, of a security is equal to

• A. the covariance between the security's return and the market return divided by the variance of the market's returns.
• B. the covariance between the security and market returns divided by the standard deviation of the market's returns.
• C. the variance of the security's returns divided by the covariance between the security and market returns.
• D. the variance of the security's returns divided by the variance of the market's returns.