此题不理解，请老师讲解一下，谢谢。

问题如下：

Drawbacks of technical analysis include which of the following?

选项：

A.It identifies changes in trends only after the fact. B.Deviations from intrinsic value can persist for long periods. C.It usually requires detailed knowledge of the financial instrument under analysis.

解释：

Trends generally must be in place for some time before they are recognizable. Thus, sometime may be needed for a change in trend to be identified.