According to the Text, in Terms of Research, "Retention Rate

According to the text, in terms of research, "retention rate" refers to

A) the length of time a research participant was able to recall information.
B) the amount of information a participant is able to learn during an intervention study.
C) how long a person holds onto negative feelings while participating in a study.
D) the percentage of participants who continued in a longitudinal study after the first year.