Code-switching: refers to the phenomenon in which speakers use multiple languages or dialects to communicate during the same conversation. Which of the following are not part of the code conversion. ["Mr. John has lived in China for many years and always insists on using Chinese when talking with his colleagues. Sometimes he really doesn't know how to express his meaning accurately, and he will be mixed with three or two English words.", 'Manager Ma received the visiting customers and introduced them to the company in Mandarin. After learning that there were two Shaanxi fellows in the customer, he immediately spoke Shaanxi and talked with them.', 'At an international academic conference, Professor Chen brought a thick Sichuan accent when he gave a speech in English, and most of the foreign scholars who attended the conference hardly understood his speech.', 'After coming to the international student dormitory, Mr. Fang, the teacher in charge of the class, asked about their daily life. When he found a student showing a puzzled expression, he immediately explained it in English.']