18. Substances change states when they move between solid, liquid, and gas forms. When a substance changes from one state of matter to another, it has undergone
A. a reduction in mass.
B. an increase in mass.
C. a chemical change.
D. a physical change.

How can antibodies "remember" a particular bacterial invader or "antigen" on that invader?

Which of the following best describes the purpose of the parathyroid hormone? a. it decreases the blood calcium level and decreases blood phosphorus level. b. it decreases the blood calcium level and increases the blood phosphorus level. c. it increases the blood calcium level and increases blood phosphorus level. d. it increases the blood calcium level and decreases blood phosphorus level.

Which of the following molecules can be broken down into simple sugars? a. nucleic acid b. protein c. lipid d. carbohydrate