Answer:
The exception here is
C2 and B2
both the bonds are pi bond in the above cases
Explanation:
The fault here is the unforeseen energy order of molecular orbitals. Since if filled, sigma 2px will be firmly pushed away by the electron density of orbitals already filled with sigma 2s (both electron densities lie on the inter-nuclear axis). Because of this, sigma 2px's energy is found shockingly greater than pi 2py and pi 2pz.
The electrons therefore occupy pi 2py and pi 2pz orbitals as opposed to normal sigma 2px (which remains vacant) while filling.
The existence of these 4 electrons in orbitals pi 2py and pi 2pz results in two pi bonds being formed.