A study found that elderly consumers were willing to pay more for a phone plan from T-Mobile than an equivalent plan from AT&T, whereas there was no difference in what younger consumers were willing to pay for the two plans. The pattern suggests that.

1. There is a main effect of consumer type (young versus old) on willingness to pay
2. There is a main effect of plan type (AT&T versus T-Mobile) on willingness to pay
3. There is an interaction between consumer-type and plan-type on willingness to pay
4. All of the above