This is the normal sequence on a dna

Question

A—C—T—G—G—A—T

Which one is a substitution?

a)A—G—T—G—G—A—T

b)A—C—T—G—A—T

c)A—C—U—G—G—A—U

d)A—C—T—G—C—G—A—T

I think it is c