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.

One should always keep in mind that psychology is essentially a laboratory science, and _________ a text-book subject. The laboratory material is to be _________ in ourselves. While the text should be thoroughly mastered, its statements should always be verified __________ reference to one's own experience, and observation of others. Prospective teachers should constantly correlate the lessons of the book ________ the observation of children at work in the school. The problems suggested for ________________ and introspection will, if mastered, do much to render practical help to find the truths of psychology.