A stock is currently trading for $30 per share. A call option on this stock with an exercise price of $20 has a premium of $12.00, and a call option with an exercise price of $40 has a premium of $2.00. What is the MAXIMUM per share dollar return on a bull call spread with these options?
A. Negative
B. $10
C. $12
D. $18
E. $20