Examples of price charges per minute by AT&T for long-distance telephone calls within the United States at different times of the day and week are as follows:

Washington Phil. D.C. Washington, D.C.

D.C. to Philadelphia to St. louis to L. A.

Peak (8am - 5pm

Monday - Friday) $0.21 $0.22 $0.24

Evenings 5pm - 11pm

Monday - Friday $0.13 $0.13 $0.14

Nights & weekends $0.11 $0.11 $0.12

a. Are there differences in incremental or outlay costs per minute for AT&T for telephone calls made during peak hours compared to telephone calls made at other times of the day?

b. Why do you think AT&T charges different prices per minute for telephone calls made during peak hours compared to telephone calls made at other times of the day?