【单选题】

{{I}}Questions 17 to 20 are based on the following passage.At the end of the passage, you will be given 20 seconds to answer the questions. Now listen to the passage{{/I}} Whether there is sufficient nitrogen on Mars is important because it_________.

A、can raise the temperature of Mars
B．can protect living beings from harmful rays
C．can change the atmosphere of Mars
D．is an important part of the air people breathe