What Additional Evidence Would Have Strengthened K

What additional evidence would have strengthened K. Werner Schaie's longitudinal study of intellectual abilities over time?

A) People who live in different geographical areas would provide evidence as to whether the results are limited geographically.
B) People with different economic backgrounds would provide insight into the effects on intellectual development variables.
C) People with differing levels of access to health care would provide insight on the effects of these variables on intellectual ability over time.
D) All of these