Using an umbrella when it rains allows one to escape from getting wet. in this case, the stimulus is the response is and the outcome is

Fossil found in strata reveal that unused body part decrease in size

The answer to this will be from your lab notes/procedure. it would be best to ask another student who participated in the lab as we do not know what was done, what enzyme or if the temperature is celsius or fahrenheit. if i were to guess though i'd say the rate stopped due to enzyme denturing at high temperature