You raise a very good point. But if we can select only from the digits 1, 2, 3, 4, 5, 6, 7, and 8, then we get a number of possible combinations that is not among the answer choices.
With this restricted set, the first space, which has to be a prime, can only be 2, 3, 5, or 7. That's 4
choices. The second space has all 8
possible digits. The third space has 8
again. The fourth space could be 2, 4, 6, or 8. 4
. That number is not offered.
So we have to assume, even if it is contrary to the test's convention, that we are allowed to use 0 and 9.
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