Thousands of years ago people guessed the time of day by watching the sun. Later，they found it was easier to tell the exact time by looking at the shadows.Thus， the sundial(日晷) was invented. The sundial proved a useful timepiece  in sunny weather. However，another typc of  timepiece was necessary  for cloudy weather and nigh time.A sand glass was another  common measurer of time.Two glass bottles were connected by a very  small opening.  The top bottle was filled with sand，which dropped slowly into the botton one in a  certain period of time.Hour glasses were widely used then.Three—minute sand glasses  are still used in many homes to time the boiling of eggs. The real ancestor of modern clocks was the water clock.In China ,an early  water clock was made up of several bowls. Water trickled(滴) from one bowl to another to keep the clock  going. Visitors to the  Beijing Palace Museum can still see the old  water clocks showing the time there. In the Middle Ages a wateless clock was invented which  worked by means of  weights. To keep  the  weight  falling  at  the  same  speed，a  system  of wheels  was  invented. The pendulum(钟摆) was  soon  developed  to control  the  speed of the  wheels. Thus the modern form of clock came into being. 9.Which of the following is the correct picturr of a sand glass? A.                       B.                       C.                     D. 10.Three minute sand glasses are still used in many homes         . A.to boil eggs                           B.to measure the time of boiling eggs C.to hold the boiled eggs                 D.to have eggs boiled 11.If you visit Beijing Palace Museum, you         . A.will see the old water clocks broken B.will find the time shown by the old water clocks is wrong C.can find the old water clocks still telling the time there D.will find the old water clocks have gone 12.In the Middle Ages the weight falling speed of a clock was kept by . A.several bowls                        B.a system of wheels C.a system of pendulum                  D.several springs