Which of the following are important challenges when applying Modern Portfolio Theory (MPT) in practice? A. Computing. power (for estimating expected returns, standard deviations and covariances) is a key challenge. B. Obtaining. precise estimates of expected returns, standard deviations and covariances is a key challenge. C. Having a large number of securities (say several hundred or more) in the inverstment universe is a key challenge. D. Option A and B E. Option A and C F. Option B and C G. Option A, B and C .