Use the Following Information for Questions

Use the following information for questions:
In a study of 900 European American children born in 1991 ("Maternal Employment and Child Cognitive Outcomes in the First Three Years of Life: The NICHD Study of Early Child Care," Brooks-Gunn, Han, and Waldfogel, Child Development 73[2002]: 1052-1072) , the authors wrote "children whose mothers worked at all by the ninth month of their life had lower [cognitive outcome] scores at 36 months than did children whose mothers did not work by that time." These results were statistically significant.
-What is the explanatory variable in this study?

A) The age of the child
B) The cognitive outcome score of the child at 36 months
C) The working history of the mother (did or did not work during the first nine months of the child's life)
D) None of the above