A scientist investigated the effect of exercise on breathing rate.
This is the method used.
1.
2.
3.
4.
Record the breathing rates of 10 male non-smokers at rest.
Tell each man to run on a treadmill at the same speed for 8 minutes.
Record the breathing rate of each man every 2 minutes.
Continue to record the breathing rate of each man for 4 minutes after he stops running
Give two variables the scientist controlled in the investigation.
(c)
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2