Answer the following question given this paragraph: 

Red blood cells are flat, round, and very small. Their small size allows easy maneuverability through the capillaries, the narrowest blood vessels, where oxygen is transferred into body cells.


Q: What are the narrowest blood vessels, where oxygen is transferred into body cells?

 Choices:

- arteries

- muscles

- viens

- capillaries

A:
capillaries
