Difference between cautious and careful

[Guest]

Test No. incompl/advan-18 "Money Matters (1)", question 9

The bank clerk at the counter did not know the customer and in view of the large amount of money involved was naturally ......... about paying out the sum on the cheque.

(a) concerned
(b) careful
(c) conscious
(d) cautious

Test No. incompl/advan-18 "Money Matters (1)", answer 9

The bank clerk at the counter did not know the customer and in view of the large amount of money involved was naturally cautious about paying out the sum on the cheque.

Correct answer: (d) cautious

Your answer was: incorrect
The bank clerk at the counter did not know the customer and in view of the large amount of money involved was naturally careful about paying out the sum on the cheque.
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Why here "cautious" of all other offers? At least two others would also have been suitable regarding the context?

[Alan]

I've used this particular adjective to show that the clerk didn't necessarily complete the transaction.
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