What stays the same when a circle is changed (2)

Circles have a unique shape. All circles look alike or you can say that they are similar figures. When you change the dimension of the diameter, d, what other dimensions are changed? However, something (a relation) stays the same. This unchanging relation is what we would like to find out. Dimensions of the diameters and circumferences of the circles are provided additionally for reference.
Knowledge assumed: Meaning of diameter, radius, and circumference.