问题如下：
The right to buy a stock only at expiration is:
选项：
A.a European put option.
B.a European call option.
C.an American call option.
解释：
B is correct. A holder of a European call option has right to buy a stock only at expiration. C is incorrect because American options can be exericsed early.
麻烦问下：A 选项 ：a European put option.不是购买一个看跌的欧式期权吗？也是一种权利呀。为什么不对？我觉得A\B 都对。
