Part A: The best inference supported by paragraph 1 of "YouTube" is that the original founders of YouTube had a vision for a platform that would grow into something much larger and more influential than they initially anticipated. The mention of how they might not have been quick to sign with Google if they had foreseen YouTube's immense success suggests that they underestimated the platform's future significance.
Part B: Please provide a specific question or options for Part B so that I can assist you further.