This is the normal nucleotide sequence on a DNA strand:
A—C—T—G—G—A—T
What is a substitution?

Select one:
a.
A—C—T—G—C—G—A—T

b. A—C—T—G—A—T
c. A—G—T—G—G—A—T
d. A—C—U—G—G—A—U