A company wants to test the effectiveness of a new acne cream. The company recruits 500 girls ages 13 to 17 who have acne. The subjects are randomly assigned into two groups. One group is given the acne cream and the other is given a placebo that looks exactly like the acne cream. Both groups apply the cream daily for two months. Facial photos are taken at the beginning and end of the treatment to compare results. Complete parts (a) through (c). (b) Identify a potential problem with the experiment design being used and suggest a way to improve it. Choose the correct answer below. A. Only girls ages 13 through 17 were used. To increase validity, use a stratified sample. B. Only one non-placebo cream was studied. To increase validity, use more creams of different varieties. C. The cream is used for two months. To increase validity, compare photos immediately after applying the cream. D. Facial photos were used to compare results. To increase validity, use video recorders to compare results