问题如下：
The longest of the widely recognized cycles was identified by Nikolai Kondratieff in the 1920s. The economic cycle has a period of:
选项：
A. 20 years.
B. 54 years.
C. 74 years.
解释：
B is correct. The economic cycle has a period of 54 years and is known as Kondratieff Wave.
老师，这道题不明白？请解释一下
