Which sentence represents a relationship between Denise's cell phone minutes and her monthly cost that results in a horizontal line?

Denise pays $0.40 per minute.

Denise pays $40 per month for the first 400 minutes, then $0.45 for each additional minute.

Denise pays $30 per month plus $0.10 per minute.

Denise pays $99 per month for unlimited minutes.

Question

Which sentence represents a relationship between Denise's cell phone minutes and her monthly cost that results in a horizontal line?

 Denise pays $0.40 per minute.

 Denise pays $40 per month for the first 400 minutes, then $0.45 for each additional minute.

 Denise pays $30 per month plus $0.10 per minute.

 Denise pays $99 per month for unlimited minutes.

anonymous · Answer

If you have a straight horizontal line, it means your dependent variable (y) stays the same no matter what. So we're looking for a plan where Denise pays the same amount of money every month no matter what. The only possible answer, then, is the last one.