Rachel is working on a project that technically allows three activities to be done at the same time. If they were to be implemented at the same time, she would need $5$ contractors in order for the activities to be completed on time. There are only $3$ contractors available for her to use. This is an example of what type of resource constraint?

Select one:

A$.$

People

B$.$

Equipment

C$.$

Information

D$.$

Materials