31. Trace determination of Pb in fish requires dry-ashing of the salted fish sample. Which of the following precautions is particularly important?
(A) The temperature should not be too high or else the Pb is volatized
(B) A Pt or Ni crucible should be used to avoid Pb adsorption from other crucible
(C) The sample should be heated to at least 700 °C to ensure complete oxidation
(D) the sample should be digested in a close environment to avoid contamination
(E) A high grade acid should be used to assist complete digestion and minimize contamination