Questions 14 to 16 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. What will be most effective about personal interviewing

A．When people to be interviewed are located in a big area.
B．When people to be interviewed are located in a mountain.
C．When people to be interviewed are located in a small area.
D．When people to be interviewed are located abroad.