B. the national popular vote plan
C. the district plan
D. the proportional plan
Answer:
This is a type of cross-sectional study.
Explanation:
This type of study is used to study various subjects during a specific moment in time. The subjects can vary on age, for example.
It is used mostly in developmental psychology and its main objective is to observe so the variables are not manipulated.
In this case, we can see Dr. Ahmandi is carrying on a cross-sectional study since she is doing research projects on five-year-olds as well as nine-year-olds to see how their memories change over time. She is doing this study in a specific moment in time to subjects with different variables.