How do simple organisms exchange substances? O A. Directly across the alveoli.
OB. Through spiracles.
C. Directly across the external membranes.
D. Through specialized gas exchange systems.

Melting glaciers is a serious threat to the environment what is the possible consequence of melting glaciers on polar bears?

Milk production during breastfeeding is increased by the suckling of a newborn from his mother's nipple. this type of feedback mechanism best describes a positive or negative

Vaccines are weakened forms of disease causing microorganisms, which are given to patients to prevent disease. after the vaccine is administered, the immune system responds by creating a(n) to recognize the a.) antibody, antibiotic b.) antigen, antibody c.)antibiotic, antibody d.)antibody, antigen