A mathematical theorem proved by one mathematician should not be accepted until each step in its proof has been independently verified. Computer-assisted proofs generally proceed by conducting a vast number of calculations -- surveying all the possible types of instances in which the theorem could apply and proving that the theorem holds for each type. In most computer-assisted proofs there are astronomically many types of instances to survey, and no human being could review every step in the proof. Hence, computer-assisted proofs involving astronomically many types of instances should not be accepted. Which one of the following is an assumption on which the argument relies?
A: Most attempts to construct proofs of mathematical theorems do not result in demonstrations that the theorems are true.
B: Computers cannot be used to assist in generating proofs of mathematical theorems that involve only a very limited number of steps.
C: The use of the computer to assist in the proof of mathematical theorems has greatly simplified the mathematician's task.
D: The use of an independent computer program does not satisfy the requirement for independent verification of each step in a proof that is extended enough to be otherwise unverifiable.
Choose A, B, C or D as your solution.
D