A controlled experiment is: 1. always conducted using only one sample group
2. one in which a scientist leads another group of scientists
3. hard to design and rarely leads to any conclusion
3. one involving two groups, where the variable being tested is the only difference

Which medullary index indicates a human hair? a. 0.653 b. 0.733 c. 0.516 d. 0.256

Why do we perceive objects as retaining their brightness under different lighting levels?

Which of the following is true of metabolism in its entirety in all organisms? a) metabolism depends on a constant supply of energy from food. b) metabolism uses all of an organism's resources. c) metabolism consists of all the energy transformation reactions in an organism. d) metabolism manages the increase of entropy in an organism.