Proofs relying crucially on computers provide less certainty than do proofs not requiring computers. Human cognition alone cannot verify computer-dependent proofs; such proofs can never provide the degree of certainty that attends our judgments concerning, for instance, simple arithmetical facts, which can be verified by human calculation. Of course, in these cases one often uses electronic calculators, but here the computer is a convenience rather than a supplement to human cognition. The statements above, if true, most strongly support which one of the following? ['We can never be completely sure that proofs relying crucially on computers do not contain errors that humans do not detect.', 'It is impossible to supplement the cognitive abilities of humans by means of artificial devices such as computers.', 'If one can corroborate something by human calculation, one can be completely certain of it.', "Only if a proof's result is arrived at without the help of a computer can one judge with any degree of certainty that the proof is correct."]