Janus Wanted to See If People Are More Attracted to Individuals

Janus wanted to see if people are more attracted to individuals with symmetrical faces compared to individuals with asymmetrical faces. Janus recruited heterosexual male and female participants for his experiment. Janus showed the participants pictures of people whose faces were modified to be more symmetrical or less symmetrical than they actually are and asked the participants to press a green button if they found the picture attractive and a red button if they did not. The participants were shown a single picture at a time and each picture was shown to them for five seconds.
-Which of the following is the independent variable in Janus' study

A) The sex of the participants.
B) The sexual orientation of the participants.
C) The button selections the participants make.
D) The photos each participant was shown.