Answer:
d. cross-sectional study
Explanation:
In research, a cross-sectional study consists of a gathering of data from a population or a subset of the population we want to investigate at an specific point of time. In other words, the data will give us an idea of what's happening within a group in a particular moment in time.
In this example, the researcher gives 20-year-olds, 40-year-olds, and 60-year-olds a questionnaire. He wants to know if older people are calmer than younger ones. We can see that <u>he wants to know what's happening within this groups (the 20 year-olds, the 40's and the 60's) during this particular time of their life </u>and therefore determine if the 60 year-olds are calmer. Therefore this is an example of cross-sectional study.
300 billion kilometers represents 2005.37613668053 AU
