The second and third rows of transition elements resemble each other much more than they resemble the first row. Explain why?

(N/A) The similarity between the second and third rows of transition elements is primarily due to the $ \text{lanthanoid contraction} $.
As we move from the second row to the third row,the $ 4f $ orbitals are filled before the $ 5d $ orbitals.
The $ 4f $ orbitals have very poor shielding effect,which leads to an increase in the effective nuclear charge.
This causes the atomic radii of the elements in the third row to be very similar to those of the corresponding elements in the second row.
For example,the atomic radius of $ \text{Zr} $ $( 160 \text{ pm} )$ is almost identical to that of $ \text{Hf} $ $( 159 \text{ pm} )$.