老师请问：
三级 2021 模考题 Equity 中，提到降低portfolio absolute risk 的方法时，用的词时adding new fund, 请问为什么不是adding new stocks with of lower covariance with the portfolio? fund 和portfolio不一样吗？fund 和portfolio什么关系？
答案原文：
The S&P 500 Index Fund is the largest contributor to total portfolio variance despite having a lower standard deviation than either the Spencer Fund or Artie Fund.
There are two potential changes that can be made to reduce the total portfolio variance (either would be acceptable):
• Kibble could add a new fund to the US equity portfolio that has a lower covariance with the portfolio than the existing funds in the portfolio.
• Kibble could replace one of the existing funds in the US equity portfolio with another fund that has a lower covariance with the portfolio than the fund being replaced.
