Which of the Following Makes It Difficult to Determine If

Which of the following makes it difficult to determine if there are gender-related differences in behavior and cognition?

A) People tend to treat children dressed as boys or girls in different, subtle ways.
B) The results show evidence for strong differences, but people don't want to accept them.
C) There are large differences between boys and girls, but highlighting those is not politically correct.
D) It is too hard to categorize people into genders for this to be studied.