Question

{"given":"Four substances are given: He (helium), Cu (copper), K (potassium), and Mg (magnesium). We need to identify which one exhibits superconductivity at approximately 4 K (the critical temperature, $T_c$).","key_observation":"Superconductivity is the phenomenon where a material's electrical resistance drops to exactly zero below a characteristic critical temperature $T_c$. The first discovered superconductor was mercury (Hg) with $T_c \\approx 4.2$ K. None of the options given (He, Cu, K, Mg) is a known superconductor at 4 K. He is a noble gas that liquefies near 4 K but does not superconduct. Cu, K, and Mg are metals but are not superconductors at any temperature under normal conditions. This question as framed has no correct answer among the options, as the intended answer (Hg) is missing. The stored answer A (He) is factually incorrect.","option_analysis":[{"label":"(A)","text":"He","verdict":"incorrect","explanation":"Helium (He) is a noble gas that liquefies at approximately 4.2 K but does NOT exhibit superconductivity. It is not a metal and cannot conduct electricity, let alone superconduct. This option is factually wrong as the stored answer."},{"label":"(B)","text":"Cu","verdict":"incorrect","explanation":"Copper (Cu) is an excellent electrical conductor under normal conditions but is NOT a superconductor at any temperature under standard pressure. It does not exhibit zero resistance at 4 K or any other temperature ordinarily."},{"label":"(C)","text":"K","verdict":"incorrect","explanation":"Potassium (K) is an alkali metal and a good conductor, but it is not a known superconductor at 4 K. Potassium does not have a critical temperature anywhere near 4 K under normal conditions."},{"label":"(D)","text":"Mg","verdict":"incorrect","explanation":"Magnesium (Mg) is a metal that was found to be a superconductor only at about 0.0002 K under extreme conditions, far from 4 K. It does not superconduct at 4 K under normal conditions."}],"answer":"None (Question is flawed — the correct superconductor at ~4 K is Hg, not listed. Stored answer A is wrong.)","formula_steps":[]}