4. The method of claim 1 , wherein the extracting the one or more n-grams using the identified key terms includes: identifying one or more terms adjacent to each key term in the knowledge unit; performing natural language processing on the one or more terms adjacent to each key term and the key terms; calculating a probability of the one or more terms adjacent to each key term in the knowledge unit as preceding or following the key term based on a function of the natural language processing; when the probability of the one or more terms being adjacent to the key term is greater than minimum threshold probability, extracting an n-gram comprising the one or more terms and the key term; and when the probability of the one or more terms being adjacent to the key term is less than the minimum threshold probability, extracting an n-gram comprising only the key term.
4. The method of claim 1 , wherein the extracting the one or more n-grams using the identified key terms includes: identifying one or more terms adjacent to each key term in the knowledge unit; performing natural language processing on the one or more terms adjacent to each key term and the key terms; calculating a probability of the one or more terms adjacent to each key term in the knowledge unit as preceding or following the key term based on a function of the natural language processing; when the probability of the one or more terms being adjacent to the key term is greater than minimum threshold probability, extracting an n-gram comprising the one or more terms and the key term; and when the probability of the one or more terms being adjacent to the key term is less than the minimum threshold probability, extracting an n-gram comprising only the key term. 5. The method of claim 4 , wherein the calculating the probability of the one or more terms adjacent to each key term as preceding or following the key term in the knowledge unit as preceding or following the key term is based on the function of the natural language processing and a frequency of occurrence of the one or more terms adjacent to each key term.