Nadia used to run a lot, but her knees have been bothering her lately. She wants to purchase a piece of fitness equipment that will help her keep up her cardio endurance levels while not putting a lot of pressure on her knees and joints. What is the BEST purchase for Nadia to make?

Question 11 options:

an adjustable plate-loaded barbell

a nautilus machine

a treadmill

an elliptical trainer