An investor can design a risky portfolio based on two stocks, A and B. Stock A has an expected return of 21% and a standard deviation of return of 39%. Stock B has an expected return of 14% and a standard deviation of return of 20%. The correlation coefficient between the returns of A and B is 0.4. The risk-free rate of return is 5%. The standard deviation of the returns on the optimal risky portfolio is :_______

a. 25.5%
b. 22.3%
c. 21.4%
d. 20.7%