In this forest there are more or less 15 kinds of snakes,
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Cfive of the snakes是很危险的
the snakes做先行词,所以用of which
======以下答案可供参考======
供参考答案1:
选C，which 引导非限制性定语从句，of which ，which指 15 kinds of snakes.
供参考答案2:
Cwhich指代15 kinds of snakes
把从句变为陈述句就好理解了：
Five of the 15 kinds of snakes are very dangerous．
供参考答案3:
C．of which
意思是 five of (the 15 kinds of snakes) are very dangerous.
逗号前是主句，逗号后是非限定性定语从句。两句之间必须有连接词啦