Answer:
Control group; experimental group
Step-by-step explanation:
Control group: In research, the term "control group" is described as a group through which specific comparisons are being made in research or experiment. The main purpose of the control group in an experiment whereby the effect of treatment is considered unknown and hence, the comparison between the experimental group and the control group are being utilized to measure the "effect of the treatment".
Experimental group: In research, the term "experimental group" determines the group of specific participants who are being exposed to an independent variable. Therefore, these participants are either exposed or receive the "treatment variable".
In the question above, the given statement represents the participants seeing "neutral faces" are in the control group and the participants seeing "smiling faces" are in the experimental group.