Answer:
c) variability, reactivity, and complexity.
Explanation:
According to the textbook, the three things that make people especially difficult to study are variability, reactivity, and complexity. Moreover, people are very difficult to study because there are ethical issues when observing and measuring other human beings. Variability makes people difficult to measure because there are many differences between people, and also people are complex beings. Reactivity makes people act differently when they know they are being observed.
