# Why are the atomic radii of iron, cobalt, nickel, and copper almost the same?

We have to select the set of elements which will have almost the same atomic radii:

1. $\ce{O, S, Se, Te}$
2. $\ce{Li, Be, B, C}$
3. $\ce{Fe, Co, Ni, Cu}$

I googled the atomic radii of the third set and found them to be extremely close to one another: all of them are nearly $\pu{125 pm}$ (according to empirical data from Wikipedia). Is there a specific reason for this, or is it just a coincidence?