TASK DEFINITION: In this task, you are given a paragraph, and your job is to generate comma-separated entities present in the given paragraph. Generate entities from a given passage in such a way that (i) they are present in the paragraph, (ii) they are non-duplicate, (iii) they underwent a state change during the process. Avoid creating entities that (i) aren't present in the process paragraph and (ii) aren't significant. 
PROBLEM: Rain clouds travel across the sky. Rain clouds are stopped or slowed by mountains or wind. The rain clouds drop the rain. The clouds are no longer raining when or if they reach the other side of the obstacle. On the other side of the mountains or rain, there isn't much rain. The area that receives little rain develops a very different ecosystem. The area that receives little rain becomes a desert. 

SOLUTION: rain clouds ,rain ,clouds ,a very different ecosystem ,desert 

PROBLEM: Two large veins bring blood to the heart. The blood enters the right atrium. The blood flows through the tricuspid valve. The blood travels into the right ventricle. The blood goes through the pulmonary arteries to the lungs. Blood picks up oxygen in the lungs. Oxygenated blood enters the left atrium. Blood goes to the left ventricle. Blood leaves the heart via the aorta to the rest of the body. The deoxygenated blood returns to the heart. 

SOLUTION: oxygenated blood ,deoxygenated blood ,oxygen 

PROBLEM: Water comes into contact with something made of iron. The water combines with carbon dioxide in the air to form carbonic acid. As the acid is formed, the iron starts to dissolve. Some of the water breaks down into hydrogen and oxygen. The free oxygen and dissolved iron bond together. They form iron oxide, which is rust. 

SOLUTION:
water ,iron ,carbon dioxide ,carbon acid ,hydrogen ,oxygen ,iron oxide ; rust