Answer: Option C.
Step-by-step explanation:
Blackworms are organisms most effeciently used by scientists to study the effect of drugs on the circulatory system. They have a transparent skin which make it easier to observe pulse rate. Drugs easily diffused through the skin of blackworms which cause immediate effect and they are easy to maintain in the laboratory. When the blackworms are incubated with extra 2 minutes, their pulse rate will increased due to excessive absorption of the test solution.