短文理解 听力原文： Dealers in government and private securities need short-term financing to purchase new securities and carry their existing portfolios of securities until those securities are sold to customers or reach maturity. Such loans are readily granted by many of the largest banks because of their high quality--often backed by pledging the dealer's holdings of government securities as collateral. Moreover, many loans to securities dealers are so short-overnight out to a few days that the bank can quickly recover its funds or make a new loan at a higher interest rate if the credit markets have tightened up. 21. Who need short-term financing according to the passage? 22.Why such loans are so high quality? 23.How long will the dealers keep such loans? (21)