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Which of the following statements concerning mortgage-backed securities are correct? 1. They are secured by a pool of residential mortgages. II. A portion of the income stream is a non-taxable retum of capital III. They are backed by the full faith and credit of the government. IV. Their maturity depends on prepayments of the mortgages in the pool A. I and III only B. I and IV only C. II, III and IV only D. I, II and IV only