challenge: What is the approximate walking speed on Mars compared to Earth?
  
  A. 2.1 km per hour on Mars, 3.9 km per hour on Earth
  B. 4.2 km per hour on Mars, 6.1 km per hour on Earth
  C. 2.5 km per hour on Mars, 4.5 km per hour on Earth
  D. 3.4 km per hour on Mars, 5.5 km per hour on Earth
  Answer: 