You will be given a definition of a task first, then some input of the task.
In this task, you are given a multiple-choice question about healthcare. Answer the question based on your information and classify your answers into '1', '2', '3', and '4'.

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:
5