NO.PZ2018062001000040
问题如下：
The primary limitation of long-term cycle theory is:
选项：
A.small sample size.
B.unobservable data.
C.hard to identify the cycle.
解释：
A is correct.
It takes very long time to complete the cycle, which leads to the disadvantage of small sample size.
我的理解是long term里不是越长能收集到的数据越多么，相对短期就少。。
