Kolari, Liu, and Zhang (KLH) (2021) proposed a novel empirical ZCAPM to capture positive and negative sensitivity to return dispersion (i.e., zeta risk). Write their empirical model. What is D_it in this model? Using the definition of D_it, write the final form of the empirical ZCAPM. Why must the expectation maximization (EM) regression method must be used to estimate this final form?