An advantage of using longitudinal studies is that a. they are relatively inexpensive b. they take little time to conduct c. they generally have small amounts of data d. subject variation is often reduced

The correct answer is option D.

Explanation:

Longitudinal study is an example of an observational research in which observations or data collected or taken repeatedly from the same subject for period of a time. These studies can extend over years or even decades.

In these type of studies less number of subjects are studied and observed.This is because subjects under observation are observed for a longer duration of time. Hence, subject variation is often reduced

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smooth muscle is composed of involuntary muscles found within the walls of organs and structures such as the esophagus, stomach, intestines, and blood vessels. those muscles push materials like food or blood through organs. unlike skeletal muscle, smooth muscle can never be under your control.