Chapter 1, Problem 7RA

Which of these statements about homeostasis is true?

$\begin{array}{l}\text{a}\text{. The internal environment is maintained absolutely constant}\text{.}\\ \text{b}\text{. Negative feedback mechanisms act to correct deviations form a normal range within the internal enviroment}\text{.}\\ \text{c}\text{. Homeostasis is maintained by turning effectors on and off}\text{.}\\ \text{d}\text{. All of these are true}\text{.}\end{array}$