问题如下：
Which of the following circumstances will most likely affect the value of an American call option relative to a European call option?
选项：
A. Dividends are declared
B. Expiration date occurs
C. The risk-free rate changes
解释：
A is correct.
When a dividend is declared, an American call option will have a higher value than a European call option because an American call option holder can exercise early to capture the value of the dividend. At expiration, both types of call options are worth the greater of zero and the exercise value. A change in the risk-free rate does not affect the relative values of American and European call options.
选项C不理解需要解释一下
