At the beginning of the year, a firm awards one stock option to its only employee. The option is awarded at-the-money when the stock is trading at $14 per share. The Black-Scholes value of the option is $6. The option vests after two years. Upon vesting, the stock is trading at $20 per share and the employee exercises the option. The par value of the firm's stock is $1 per share. What is the yearly stock option compensation expense over the vesting period?

a. $7

b. $10

c. $3

d. $6