Dr. Paulson conducts an evaluation of an anti-poverty program among single mothers in Chicago. She finds that providing two six-hour sessions per week of free group child care reduces the likelihood that single mothers apply for federal benefits. When the program expands to Baltimore and San Antonio, Dr. Paulson does not find the same results. Which of the following factors might explain the low external validity in the first study?

a. The questions about federal benefits varied between the first study and the second study.
b. The researchers did not find enough single mothers in Baltimore and San Antonio.
c. Single mothers in Baltimore and San Antonio may have stronger networks of friends and family who can watch their children and thus the program benefit is minimal to them.
d. Single mothers in Baltimore and San Antonio were embarrassed to admit that they were receiving federal benefits.