Of course, a sundial did not work at night or on cloudy days, so men kept inventing other ways to keep track of time. After glass blowing was invented, the hourglass came into use. An hourglass is a glass container for measuring time in which sand moves slowly from the top half to the bottom in exactly one hour. The hourglass is turned over every hour so the sand could flow again.
One of the first clocks with a face and an hour hand was built for a king of France and placed in the tower of his palace. The clock did not show minutes or seconds. Since there were no planes or trains to catch, people were not worried about knowing the exact time. Gradually, clocks began to be popular and unusual. One clock was in the shape of a cart with a horse and driver. One of the wheels was the face of the clock.
Today, scientists have invented clocks that tell the correct time to a split second. Many electric clocks are often made with built-in radios, which can sometimes be set to turn on automatically. Thus, instead of an alarm ringing in your ear, you can hear soft music playing when it is time to get up. Some clocks will even start the coffee maker!
In the first paragraph, the word "sundial" refers to ______.

A. the shadow of the sun
B. the circle on the ground
C. a tool to carry stones
D. a timekeeper