Which of the following relations is a function?
{ (-2, 4), (-1, 3), (5, 2), (1, 4) }
{ (1, 7), (1, 8), (2, 7), (3, 8) }
{ (2, 4), (4, 6), (4, 8), (6, 4) }
{ (5, 7), (6, 3), (8, 6), (5, 4) }

Aparticular telephone number is used to receive both voice calls and fax messages. suppose that 25% of the incoming calls involve fax messages, and consider a sample of 25 incoming calls. (a) what is the expected number of calls among the 25 that involve a fax message?