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答案详解如下：

Q59.  B    Study Session 12-53.f

The average covariance of a stock's return with returns of all other stocks in a portfolio is the most important factor to consider when adding anew stock to a portfolio. Adding a stock whose returns are less than perfectly positively correlated with the returns of the other stocks in the portfolio will cause the total standard deviation of the portfolio to be lower than the weighted average of the components' standard deviations. This is the benefit of diversification.