Answer:
Closure of the mitral and tricuspid valves, and closure of the pulmonic and aortic valves.
Explanation:
The first sound results from closure of the mitral and tricuspid valves, which separate the upper and lower chambers of the heart. The second sound occurs as a result of closure of the pulmonic and aortic valves. Blood travels across these valves on its way to the lungs and heart.
