Why should a breakable, mercury-filled thermometer NOT be used in a lead-acid battery to measure electrolyte temperature?

Correct Answer: Option C — accidental breakage can cause severe sparking and explosions

Mercury contacting battery components creates an amalgam that destroys internal lead structures, resulting in violent exothermic reactions, severe hydrogen ignition, and explosions. USCG safety standards and IEEE 45 prohibit glass or mercury instruments near open-cell installations.