Why do metals generally have high melting points, whereas mercury and gallium have exceptionally low melting points?

Options:

• A: Mercury and gallium have completely filled d-orbitals which participate strongly in covalent bonding
• B: Mercury and gallium have weak metallic bonding due to the shielding effect and stable electronic configurations that prevent effective sharing of valence electrons
• C: Mercury and gallium are non-metals that show metallic luster only under specific conditions
• D: Mercury and gallium have high ionization energies which prevent them from forming any metallic lattice

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