In 1932, every child born in Scotland in 1921 was given an intelligence test. At age 80, the survivors were tested again. Finally, at age 90, the remaining survivors were again retested. This investigation exemplifies a study.

longitudinal study.

Explanation:

Longitudinal study are said to be stidies that follow one set of participants over time, using research methods such as experiment, survey or observation.

The important points about longitudinal studies are it makes use the same participants, and its measure are taken over time, to enable proper comparisons. This type of study can last moths or years.

One major strength of such studies is that Longitudinal studies are useful for looking at developmental trends. They are majorly used to see how an individual affects certain characteristics.

the best answer is b.

hope this : )

b) to encourage free trade