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. The paper industry needed wood pulp in the early 1800s because ______.
A．wood pulp was of good quality B．wood pulp was very cheap C．rags were not enough D．rags could not be obtained in the U. S.