Why the rarest word is the right blank
Most cloze tools remove a word at a fixed interval — every seventh word, say. That gives you questions like “The mitochondrion is ___ site of oxidative phosphorylation”, where the answer is “the”. You can fill that in from grammar alone, without having read a line of the chapter, so the question measures nothing.
Rarity fixes that. Across a page of biology notes, “the” might appear forty times and “phosphorylation” twice. The word that appears twice is the one carrying the specific claim of the sentence, and it is the one that goes missing in the exam hall. Blank that, and the question has something to test.
This is the same statistic that decides which words count as keywords in a document: terms that are frequent in this text and rare in ordinary writing. Running it over your notes produces blanks for terminology, proper nouns, dates and quantities — close to what the short-answer sections of most Indian university papers ask for.