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.