I want to know the answer to the following question:
Billions of dollars have been paid by which companies?
All the background I'm provided with is that "An investigation by ProPublica found that at least 21 doctors have been paid more than $500,000 for speeches and consulting by drugs manufacturers since 2009, with half of the top earners working in psychiatry, and about $2 billion in total paid to doctors for such services.". Is that enough to answer the question?
Answer: no