An increase in which of the following will increase the value of an American call option? 1. Dividend payments of the underlying asset 2. Rate of risk-free asset 3. Time to expiration 4. Variance of the return on the underlying asset O a. All are correct O b. 1, 2, 3 are correct O c. 1, 3, 4 are correct O d. 2, 3, 4 are correct e. 1, 2, 4 are correct