问题如下：
Which of the following statements regarding sector indices is most accurate? Sector indices:
选项：
A.track different economic sectors and cannot be aggregated to represent the equivalent of a broad market index.
B.provide a means to determine whether an active investment manager is more successful at stock selection or sector allocation.
C.apply a universally agreed upon sector classification system to identify the constituent securities of specific economic sectors, such as consumer goods, energy, finance, health care.
解释：
B is correct.
Sector indices provide a means to determine whether a portfolio manager is more successful at stock selection or sector allocation.
请老师解释下三个选项的意思，和正确答案的知识点
