In which kind of study does an investigator study different groups of people, of different ages, at the same time?

A) cross-cultural
B) cross-sectional
C) longitudinal
D) factor analysis

A cross-sectional study involves studying different groups of people of different ages at the same time. This allows for comparisons between age groups and can provide information about age-related changes in a certain trait or outcome. Cross-cultural studies involve comparing different cultures, longitudinal studies involve following the same group of individuals over a period of time, and factor analysis involves identifying patterns or relationships between different variables.