An echocardiogram is a type of ultrasound test that measures sound waves as they bounce off the different parts of a person's heart. Which of these is the best model for how this test works? A. Counting the seconds between yelling and hearing an echo in a cave B. Counting the seconds between seeing lightning and hearing thunder C. Counting the minutes it takes a pot of water to come to a boil D. Counting the minutes it takes a ship's anchor to reach the seafloor