【单选题】

Questions 14 to 17 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.
What can an exhaust system do if it is efficientA．It can prevent the waste gas from escaping.
B．It can let the waste gas and compound escape quickly out of it.
C．It can prevent the engine from producing waste gas.
D．It can store as much waste gas as possibl