D, the independent variable is what you are changing
Cell,tissues ,Organs and Organ Systems. After tissues, organs are the next level of organization of the human body. An organ is a structure that consists of two or more types of tissues that work together to do the same job. Examples of human organs include the brain, heart, lungs, skin, and kidneys
Four unifying principles form the foundation of modern biology<span>: cell theory, evolutionary theory, the gene theory and the </span>principle<span> of homeostasis. These </span>four principles<span> are the founding principles of each category of biology</span>