Answer:
Inference context clue
Explanation:
Inference context clue was used in this sentence because inference context clues help the reader use his power of reasoning to deduce the meaning of words.
In the sentence, the meaning of the word prototype was not specified but when the reader sees that the prototype was used by the builders to produce thousands of something else, he would have the idea that a prototype is a model that can be copied to produce more of something.
