Assume two securities are negatively correlated. If these two securities are combined into an equally weighted portfolio, the portfolio standard deviation must be:
a. equal to the standard deviation of the overall market.
b. equal to zero.
c. less than the weighted average of the standard deviations of the individual securities.
d. equal to or greater than the lowest standard deviation of the two securities.
e. equal to the arithmetic average of the standard deviations of the individual securities.