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 minutes it takes a ship's anchor to reach the seafloor
C. Counting the seconds between seeing lightning and hearing thunder
D. Counting the minutes it takes a pot of water to come to a boil