A geneticist is conducting an experiment by making a testcross. She expects the offspring of the testcross to result in a 1:2:1 ratio. She wants to see if her data for the three phenotypic classes could be reasonably assumed to have deviated from the expected values by chance. How many degree(s) of freedom should she use when evaluating the chi-square goodness-of-fit test?

a. 1
b. 2
c. 3
d. 4
e. 5