Which relation is a function?

a.
{–2, 10), (–6, 10), (–2, 3)}

b.
{(0, 3), (0, 9), (12, –4)}

c.
{(5, 5), (5, 9), (5, 15)}

d.
{(–5, 1), (4, 9), (6, 10)}

Lizette is training for a marathon. at 7: 00 she left her house and ran until 8: 30, then she walked until 11: 30. she covered a total distance of 18 miles. her running speed was six miles per hour faster than her walking speed. find her running and walking speeds in miles per hour.