The Best Reason to Use a Mechanical Device, Such as an Actometer

The best reason to use a mechanical device, such as an actometer, is that

A) it is easier to use with children than using questionnaires.
B) it permits the researcher to be free to measure other things in the study.
C) mechanical devices can assess a wide range of overt and covert behaviours.
D) it is free from biases associated with human raters.