Questions 18 to 20 are based on the following passage. At the end of the passage, you will be given 15 seconds to answer the questions. Now listen to the passage. When did Daylight Saving Time become observed nationally

A．In the volcanoes.
B．On the floor of the sea.
C．Under the river bed.
D．At the foot of the mountains.

A．people wanted higher quality paper
B．paper made from rags deteriorated too quickly
C．wood pulp had become too expensive
D．the supply of rags was insufficient