【单选题】

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.
What’s the function of the mirror in the experimentA．To enable the speaker to dress himself up in the experiment.
B．To enable the speaker to take a picture of himself during the experiment.
C．To enable the speaker to get more lights in the experiment.
D．To enable the speaker to observe his actions in the experiment.