Task: Question Answering
Input (English): Question: A selective calcium electrode is characterized by having: 
 Options:  <1> A membrane that is a solid crystal. <2> An internal solution of 0.1 M HCl <3> An internal silver electrode. <4> A membrane of silver salts. <5> An internal aqueous solution of CaCl 2 saturated with AgCl.
Output (English): 5