Do predators really kill only the old and sick prey?

No, many predators kill young and healthy prey, as well as old and sick. It really just depends on the type of predator and the situation. A pack of wolves may prey on an old elk because they are small and that is easier for them to kill, while a lion may prey on a young gazelle.

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