The correct answer is B. Participants who drop out during the course of the study may be different in important ways from ones who did not drop out.
Explanation:
In longitudinal studies, the same population or participants are studied for a long time, usually multiple years. This method helps the researcher to get meaningful results about participants' ideas, behaviors, etc. and how these change over time.
However, this type of study has also disadvantages, one of these is that participants are more likely to drop out because it requires more time than participating in regular studies. Moreover, this can cause important information to be omitted as those participants who drop out might offer data that is important and unique. According to this, the correct answer is B.