Emma is planning an experiment to examine whether reading to children increases their vocabulary size. She plans to measure the vocabulary size of a group of 18-month-olds, read to them three times a week for three months, then measure their vocabulary size again. How could Emma change the design of the study to remove a major threat to internal validity?
a) She could add a comparison group.
b) She could make it a double-blind placebo control study.
c) She could counterbalance her conditions.
d) She could add a manipulation check.