A big University wanted to determine whether a wellness program that focused on walking to enhance employee health would cut down on employees$$ use of health benefits. Employees in the Liberal Arts Department were invited to participate and received a $$50$ gift card to Best Buy for taking part in the program. Employees in the Engineering Department did not participate in the program. Health benefits data were examined for both departments. The Engineering Department employees described in this scenario could be labeled the

a$.$ control group.

b$.$ survey group.

c$.$ case group.

d$.$ treatment group.