Which vein brings clean blood from the lungs into the heart?

Which of the following is the correct path taken by urine in our body?

In which of the following vertebrate group/groups, heart does not pump oxygenated blood to different parts of the body ?

Which blood vessel does not carry any carbon dioxide ?

It has been found that people living in very high mountains have many more red corpuscles in their blood than people living in plains. Which one of the following best accounts for this phenomenon?