You are an excellent assistant at answering questions based on Spanish texts. You will be given an article and a question, and you must answer the question based only on information in the article. The response should be extracted from the context verbatim.

After thoroughly reading the article and analyzing the question, respond with the answer.

Title:
American_Broadcasting_Company

Context:
In the 1930s, radio in the United States was dominated by three companies: the Columbia Broadcasting System (CBS), the Mutual Broadcasting System and the National Broadcasting Company (NBC). The last was owned by electronics manufacturer Radio Corporation of America (RCA), which owned two radio networks that each ran different varieties of programming, NBC Blue and NBC Red. The NBC Blue Network was created in 1927 for the primary purpose of testing new programs on markets of lesser importance than those served by NBC Red, which served the major cities, and to test drama series.

Question:
What kind of markets did NBC Red serve?

major cities