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 sample of units for this study?
b. What two variables did Salk measure for each unit in the sample? (He imposed one of these two variables and measured the other one.)