What Is a Major Qualification of the Research Findings on Gender

What is a major qualification of the research findings on gender differences

A)  Most of the research has been conducted on white middle-class children. 
B)  The data are indicative of group differences and tell us little about individuals. 
C)  The findings are based on only a few studies and are therefore not very representative of the population. 
D)  There is wide variation from study to study in how the variables have been operationalized.