Task: Question Answering
Input (English): Question: A calomel electrode: 
 Options:  <1> It is a mercury indicator electrode. <2> It is a reference electrode that contains nitrate ions. <3> It is an indicator electrode for nitrate ions. <4> It is a reference electrode that uses silver and silver chloride. <5> It is a reference electrode that contains a saturated solution of mercury chloride (I).
Output (English): 5