23. Economic systems

natural systems.

a. operate within

b. are independent of

c. have no relation to

d. are equal to

Option A

Explanation:

Most of the human economic systems if not all operate with the human natural systems. Human economic system and natural systems almost dependent on one another in that they are linked by a network of interconnected  processes; for example, human economic activity depend on natural, including a maintained system of primary productivity such as forests and agriculture etc.

the oxygenated blood then leaves the lungs through pulmonary veins, which return it to the left heart, completing the pulmonary cycle. this blood then enters the left atrium, which pumps it through the mitral valve into the left ventricle. from the left ventricle, the blood passes through the aortic valve to the aorta.

deoxygenated blood leaves through the right ventricle through the pulmonary artery. from the right atrium, the blood is pumped through the tricuspid valve (or right atrioventricular valve), into the right ventricle.