Part A:
The inference best supported by paragraph 1 of “YouTube” is:
4. The founders of YouTube may regret their decision to sell to Google.
Part B:
The sentence from paragraph 1 that best supports the answer to Part A is:
4. If the original YouTube founders had had a crystal ball, they might not have been so quick to sign on the dotted line!
This sentence suggests that the founders may have regrets about their decision, implying that they would have made a different choice if they had foreseen the outcomes.