The four statements are about the movement of blood during one complete circuit of the body.
1 Blood flowing from the lungs flows through the heart twice.
2 All the blood flowing from the lungs flows through the kidneys.
3 All the blood flowing from the liver flows through the lungs.
4 Only blood flowing from the digestive system flows through the liver.
Which of the above statements are correct?

Answer:

21) B

1 – Correct
Blood from lungs flows to the heart. The heart then pumps the blood to the rest of the body. From the body, blood then flows back to the heart followed by the lungs. So the blood from lungs flows through the heart twice before coming back to the lungs.

2 – Wrong
Lungs receive all the blood from the heart. When the heart then pumps blood to the body, it is distributed to different parts. Hence, kidneys do not receive all of the blood that flows to the lungs.

3 – Correct
Lungs receive all the blood that flows in the liver.

4 – Wrong
The liver can obtain blood from two ways.

Statements 1 and 3 are correct. Hence, B is correct.