Burtonian line is seen in poisoning by -

Ans. is 'a > b & c' i.e., Lead > Mercury & Copper * Burtonian line is blue line, which is seen on upper gums in lead poisoning.* Similar blue line (but not called burtonian line in these cases) is also seen in poisoning with Mercury (Hg), Copper (Cu), Silver (Ag), bismuth (Bi), and Iron (Fe).