In a cross-sectional study related to developmental psychology:

The right answer is "Different age groups are tested at the same time". This type of research is known as A Cross-Sequential Design. These investigations have a cross-sectional and longitudinal, involves analyzing a group of people of different ages (For example 10, 20, 30 and 40 years) over time (For example, for 40 years), this in order to study human development in a particularly long period of life.

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