Theo was doing research to support the claim that exercise increases heart and lung health. He had 50 people do a rigorous exercise program. Each week, for six months, he tested their heart and respiratory rate. He determined that exercise does increase heart and lung health. Which best describes his claim?

It is valid because he used multiple trials.
It is invalid because he did not have a control group.
It is invalid because his sample size was small.
It is valid because he controlled the exercise program.