There are two semicircles in the drawing: the arc going clockwise from a to c and the arc going counterclockwise from a to c.
AOB is adjacent to BOC
BOC is adjacent to COD
and
COD is adjacent to AOD
Since COD is 45 degrees, angle AOD is 180 degrees - 45 degrees or 135 degrees.
The circumference of the circle is pi * AC. The length of the arc AD = 135/360 * pi * AC = 1.18 * AC.
Answer: If AC is a diameter, which of the following must be a semicircle? ABC
Hope this helps(:
