Mammalian Heart Structure and Function Steps:

• Identify key features: mammalian heart has four chambers with right side handling deoxygenated blood and left oxygenated.
• Evaluate statement 1: correct, as right atrium receives deoxygenated blood from body.
• Evaluate statement 2: incorrect, left ventricle does not pump to lungs (right does).
• Evaluate statement 3: correct, semilunar valves prevent backflow from arteries.
• Select option matching correct statements 1 and 3.

Why B is correct:

• Matches statements 1 and 3, aligning with heart's double circulation where right side sends blood to lungs and valves ensure unidirectional flow.

Why the others are wrong:

• A includes incorrect 4 (atria do not have thick walls like ventricles).
• C includes incorrect 2 (pulmonary artery carries deoxygenated blood, not oxygenated).
• D repeats incorrect 2 and 4.

Final answer: B