I want to know the answer to the following question:
What mode of public transportation in New Haven phased out the trolley service in the 1920's and 1930's?
All the background I'm provided with is that "In the 1920s and 1930s, some of the trolley lines began to be replaced by bus lines, with the last trolley route converted to bus in 1948.". Is that enough to answer the question?
Answer: yes