(a) 7. Right ventricle, 2. Pulmonary arteries
(b) Valves - They ensure that blood does not flow backwards when atria or ventricles contract.
(c) Left and right Ventricles (6 and 7) have thicker walls as compared to left and right atria (8 and 9). They have to pump blood out of the heart. They pump blood at a higher pressure to ensure it reaches all parts of the body.
OR
(i) 8. Left atrium, 3. pulmonary veins
(ii) The separation of the right and left side of the heart is useful to prevent mixing of oxygenated blood and deoxygenated blood.