Given a text T, and several options, according to the question posed in the text T to choose the most appropriate option from the options as the answer, please directly output the most appropriate option, do not need to output the explanation.

What is a major problem with long-term cycle theories?
Option A:The sample size is small.
Option B:The data are usually hard to observe.
Option C:They occur over such a long period that they are difficult to discern.

Answer: A