Ramer wants to find out how pressure affects the force of gravity acting on dust particles in a cloud. Tamer designs an experiment in which he places dust particles in a pressure chamber. He is able to adjust the pressure in the chamber from 0 Pascals to 780 Pascals. What should Tamer use as his control group? 1. A sample of dust particles at 0 Pascals 2. He does not need a control group. 3. A sample of dust particles at 780 Pascals 4. A sample of dust particles in Earth's atmosphere