What Does the Value of Kw Say About the Extent

What does the value of Kw say about the extent to which water molecule react with themselves?

A) Because the value of Kw is so small, the extent to which water ionizes is quite large.
B) Because the value of Kw is so small, the extent to which water ionizes is quite small.
C) Water molecules react with themselves at a constant rate, never increasing or decreasing.
D) When an acid is added to water, the value of Kw will decrease.