The option that expresses the main idea of this passage is: a. A computer is a high-speed adding machine.
The main idea that is being promoted by the author of the text is that the computer is a high-speed adding machine.
Even though many believe that the computer is superior when compared to man's brain, this is not true as the computer only gives out what it gets when instructions are provided.
So, the option that expresses the main idea of the passage is option A.
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