Questions 4 to 7 are based on the following conversation. At the end of the conversation, you will be given 20 seconds to answer the questions. Now listen to the conversation.

Each of those machines can only perform ______.

A．Wearing arm weights while you are swimming.
B．Jogging vigorously in one place for a long time.
C．Using bicycles that require you to use both your arms and legs.
D．Walking slowly while swinging your arms back and forth.

A．Around 1600.
B．Around 1700.
C．Around 1800.
D．Around 1900.