What type of pattern recognition starts with features and gradually builds up to more complex structures, such as letters, words, and phrases?

A) top-down processing
B) knowledge-driven processing
C) bottom-up processing
D) both a and b

Bottom-up processing starts with the analysis of individual features and builds up to more complex structures. In contrast, top-down processing and knowledge-driven processing involve starting with higher-level knowledge and expectations and using that to guide perception and recognition. So, option C is the best choice for this question.