A researcher is examining cooperation between peers by observing children at six years eight years and 10 years old if the researcher uses a cross-sectional design and obtains 20 scores for each age...

A researcher is examining cooperation between peers by observing children at six years eight years and 10 years old if the researcher uses a cross-sectional design and obtains 20 scores for each age how many children participated in the entire study? A. 40 B. 20 C. 60 D. 120