A biological system is a complex network of biologically relevant entities. ... Examples of biological systems at the macro scale are populations of organisms. On the organ and tissue scale in mammals and other animals, examples include the circulatory system, the respiratory system, and the nervous system. source: google
F is the correct answer I think
Mitosis should be the answer. if not that, then cell division. hope this helps :3
Answer:
1. hydrogen; 2
2. it means two strands of DNA twisted around each other like a winding staircase
Cells, tissue, organs, organ systems, and organisms.