Although Masters and Johnson Are Often Credited as the First

Although Masters and Johnson are often credited as the first to use observational techniques in sex research, there were a number of other researchers who had used this method. What was the actual reason that their techniques were innovative?

A) They enabled researchers to understand the prevalence of certain sexual responses.
B) They were the first to use direct observations of cervical contractions and blood flow changes during orgasm in a female patient.
C) They were the first to observe mechanical interactions between the penis and vagina.
D) They enabled researchers to measure sexual response.