<span>A. Mammals use a closed, double-loop circulatory system aided by a four-chambered hearts.
The four chambered heart of a mammal is divided into two atria and two ventricles. Double-loop circulatory system is used. One loop for oxygenated blood, the other loop for deoxygenated blood.</span>
Six <span>characteristics are used to describe living things. </span>To be considered alive, an object must exhibit all of the characteristics<span> of </span>living things<span>.</span> B. 6
