Evaluations for input

A researcher was interested in whether or not jazz vocals and opera influence men and women emotional states. She hypothesized that these types of music influence men and women differently. In a study investigating this hypothesis, 40 men and 40 women heard a jazz piece, and 40 men and 40 women heard an operatic piece. The jazz piece was sung by a man, and the operatic piece was sung by a woman. Afterward, participants rated themselves on an inventory measuring emotional state. Higher scores on the inventory indicate positive mood. Results of this study are represented in the graph below. <image 1> Which of the following is the most serious problem with the methodology of this research?

(A) Men and women did not listen to both types of music

(B) The singers were not the same gender

(C) The sample size was too small to draw a valid conclusion

(D) The participants were not musicians

Answer with the option's letter from the given choices directly. No punctuation.