I want to know the answer to the following question:
What materials were 45 rpm records made of?
All the background I'm provided with is that "The short playing time of a single 45 rpm side meant that long works, such as symphonies, had to be released on multiple 45s instead of a single LP, but RCA claimed that the new high-speed changer rendered side breaks so brief as to be inaudible or inconsequential.". Is that enough to answer the question?