In the following passage, some of the words have been left out. Read the passage carefully and select the correct answer for the given blank out of the four alternatives.

Left to itself, then, attention  _____________ obeys natural laws and follows the line of least  resistance. _______________ far the larger portion of our attention is  of this type. Thought often runs on hour after hour when we are  neither conscious of effort _____________ struggle to compel us to cease thinking about this thing and begin thinking about _________.  Indeed, it may be doubted ___________ this is not the case with some persons for days at a time, instead of hours. The things that present themselves to the mind are the things which occupy it; the character of the thought is determined by the character of our interests.