Please help me solve these Finance problems, STEP-BY-STEP, please. Thank you very much!!

10. The 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 weight of that security. C. the variance of the security's return divided by the standard deviation of the market's returns. D. the variance of the security's returns divided by the variance of the market's returns