Brian Wants to Study How Toddlers Tackle Difficult Situations by Themselves

Brian wants to study how toddlers tackle difficult situations by themselves in their parents' absence.While observing the children in a playground,Brian helps Timmy,a two-year-old,tie his shoelace.This is an example of

A) research participant bias.
B) experimenter bias.
C) placebo effect.
D) double-blind experiment.