Questions 18 to 20 are based on the passage you have just heard.

　　18. A) To examine the chemical elements in the Ice Age.

　　B) To look into the pattern of solar wind activity.

　　C) To analyze the composition of different trees.

　　D) To find out the origin of carbon-14 on Earth.

　　19. A) The lifecycle of trees.

　　B) The number of trees.

　　C) The intensity of solar burning.

　　D) The quality of air.

　　20. A) It affects the growth of trees.

　　B) It has been increasing since the Ice Age.

　　C) It is determined by the chemicals in the air.

　　D) It follows a certain cycle.