In a review of 2,000 studies of human behavior that date back to the 1940s，two Swiss psychologists, declaring that since most of the studies had failed to control for such variables as social class and family size, none could be taken seriously.
A.psychologists, declaring that since most of the studies had failed to control for such variables as social class and family size,
B.psychologists, declaring that most of the studies failed in not controlling for such variables like social class and family size，and
C.psychologists declared that since most of the studies, having failed to control for such variables as social class and family size,
D.psychologists declared that since most of the studies fail in controlling for such variables like social class and family size,
E. psychologists declared that since most of the studies had failed to control for variables such as social class and family size,
