Answer:
C is Wrong there was 1 billon people in 1804 A is Also Wrong B i also wrong so its D
Explanation: Can i have Brainliest
A I believe should be correct
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