问题如下：
Give two examples of situations where an investor might want to use stock index futures.
选项：
解释：
Possible examples: an investor wants to be out of the market for a period; an investor wants to change the beta of his or her portfolio; an investor specializes in picking stocks but does not want an exposure to the return on the market portfolio.
an investor specializes in picking stocks but does not want an exposure to the return on the market portfolio. 老师最后一句话怎么理解，股指期货是无法锁定特定风险的吧？
