Answer: a. Rule out other explanations such as an error by the computer distributor.
Explanation:
Accusations without proof can be a very dirty affair as it can soil reputations if proved to be false both of the accuser, and the accused.
For this reason, it is best that you eliminate all other sources of error before you act on the suspicion that Bill has something to do with the missing computer.
These sources of error include an error from the Computer Distributor so Option A is correct.