What prevents blood from circulating backward in veins? A. valves
B. capillaries
C. lungs
D. heart

When should a bar graph be used? a. when there is no independent variable. b. when the independent variable is continuous and does not show a relationship to the dependent variable. c. when the independent variable is continuous and shows a causal link to the dependent variable. d. when the independent variable is composed of categories and does not show a relationship.

The table below shows the number of foot bones in some horse fossils. horse fossil recordhorse fossil number of foot bonesp 19q 17r 24s 13ancient horses had more bones in their foot then present-day horses. the present-day horse has 11 foot bones. what is the correct order of evolution of the horse starting from the youngest fossil? a. q p r s b. s r p q c. s q p r d. r p q s

Which of the following statements best distinguishes a hypothesis from a theory? a. theories are hypotheses that have been proved. b. hypotheses are guesses; theories are correct answers c. hypotheses usually are relatively narrow in scope; theories have broad explanatory power. d. theories are proved to be true; hypotheses are often contradicted by experimental results.