The market risk beta of a security is equal to _____

A、Theco-varianceofthe security'sreturnsandmarketreturnsdividedbythevarianceofthemarket's returns;
B、Theco-varianceofthesecurity'sreturnsandmarketreturnsdivided bythestandarddeviationofthemarket'sreturns;
C、Thevarianceofthesecurity's returnsdividedbyitsco-variancewiththemarket'sreturns;
D、Thevarianceofthesecurity's returnandmarketreturndividedbythevarianceofmarketreturn