Definition: 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'.
Input: Question: Sulfonic acids are converted to sulfonyl chlorides by treatment with: 
 Options:  <1> Methylene chloride <2> Chlorine. <3> Sodium chloride. <4> Thionyl chloride. <5> Chloroform.
