问题如下：
Which of the following types of style analysis use(s) a bottom-up approach to estimate the risk exposures in a portfolio?
选项：
A.Returns-based style analysis only
Holdings-based style analysis only
Both return-based and holdings-based style analysis
解释：
B is correct.
Holdings-based style analysis estimates the portfolio’s risk exposures using the securities held in the portfolio (a bottom-up approach), whereas returns-based style analysis uses portfolio returns to estimate a portfolio’s sensitivities to security market indexes (a top-down approach).
老師 這題說holding based 就是bottom up，但在Equity這題NO.PZ2019012201000060
，助教說不能holding base不一定是bottom up, return based也可以。請問這兩個怎麼區別？
