A：

Ultrasound M is a collection of ultrasound A signals during a period of time. So, it is a one-dimensional image. Adopt the gray modulation method. Then all interfaces in different depths will produce echoes. Mark these echoes with highlighted points on the vertical scanning line of display. When the organ is moving, points on the vertical scanning line change their positions to sample these echoes at an interval and show them line by line on the screen chronologically. The curve distance changes with the movement of organ. If one interface in the organ is static, its curve will be a straight line.