Mental models guide our perceptions and help us make predictions. Most of our mental models are built　1　the structure of our nervous systems, and we are usually　2　of them. 　　We　3　the world not according to direct knowledge of reality, but according to mental models, which people often mistake　4　reality. For example, we all share a built-in mental model that the world is continuous,　5　our eyes tell us differently. This built-in mental model tells us what to "see" in a part of the eye that doesn’’t　6　see anything. This "blind spot" model causes us to　7　made-up information (a continuation of surrounding patterns ) and　8　that if we look at a previously hidden spot, it will look like its　9　. 　　Not all mental models are built in; some we learn or　10　. For example, most people believe that the automobiles driving down the street will not turn　11　the sidewalk and hit pedestrians. If we believe　12　, we would act like　13　squirrels, always stopping to look around and proceeding　14　no automobiles were operating nearby. 　　We use mental models of our surroundings to perceive what we believe to be　15　and to predict what may happen. These mental models　16　to our surroundings--we have mental models of objects ,of the environment ,and of other people--and to our own capabilities and　17　 　　All of our mental models are, unfortunately, approximations　18　in the lump of nervous tissue we call the brain. Sophisticated　19　the brain is, it is very small and compared with the　20　of the outside world.

A． unless
B． when
C． if
D． though