Refer to Case Study 1.1, Heart or Hypothalamus? in which Lee Salk exposed newborn infants to the

Question:

Refer to Case Study 1.1, “Heart or Hypothalamus?” in which Lee Salk exposed newborn infants to the sound of a heartbeat or to silence, and then measured their weight gain at the end of 4 days.
a. What is the explanatory variable in Salk’s study?
b. What is the outcome (response) variable in Salk’s study?