Directions: Replace the number with the word(s) that best fit(s) the meaning of the passage as a whole. Choose the best option for the blank marked (46).
It is _____ (46) to read a book about our planet by an author who does not allow facts to be _____ (47) by politics: well aware of the political disputes about the effects of human activities on climate and biodiversity, this author does not permit them to _____ (48) his comprehensive description of what we know about our biosphere. He emphasizes the enormous gaps in our knowledge, the _____ (49) of our observations, and the _____ (50) of our theories, calling attention to the many aspects of planetary evolution that must be better understood before we can accurately diagnose the condition of our planet.