问题如下：
According the data analysis, The share price of Stock A has reached $39 three times during the last month, and then it fell back every time. $39 can be described as
选项：
A.change in polarity.
B.a support level
C.a resistance level
解释：
C is correct. Resistance level is where the share prices tend to find resistance when it rises. Once $39 is hit, stock price fell back every time, therefore selling is expected in order to prevent price decreasing.
请问 这是哪个知识点？
