Mutations that change an amino acid codon to a stop codon are called

Nonsense mutation is the correct answer.

glad i could help(:

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