Answer: experimental
Explanation:
In a research design such as the one described in the scenario above, participants are separated into two distinct groups. The hypothesis (if swimming with paddles is faster than without paddles) being tested is applied on one of the two groups while the other group remains as before. Hence, the group which receives the treatment which is to be tested (those who swim with paddles) are called the experimental group. The other group of participants who continue to swim without paddles are the control group.
Hence, the other group of three member who were asked to swim with paddles for two weeks are the experimental group because they are the ones who received the actual treatment in the experiment.