UCAT DM syllogisms

hello, I came across this question while i was doing skill trainers on medentry.

At least one fo the whales is a crub.
All crubs are not octopuses.

For a statement: 'Some octopuses are not crubs' the correct answer is No. Could anyone explain why is that?

When it says some it means that more than one but not all curbs are not octopuses, implying that there is a possibility of at least one curb being an octopus. However, the statement explicitly says curbs are not octopuses, so the possibility of a curb being an octopus 0 - causing the statement to be false.

Thank you for your answer. However, I have just realised that I made a mistake. The stimulus was saying :
At least one of the Whales is a Crub. All Crubs are Octopuses.

The mistake was that I put ' all crubs are not octopuses' which in my opinion would mean that 'no crub is an octopus', but the stimulus says that 'every crub is an octopus'. Sorry about the misunderstanding From my reasoning, only because every crub is an octopus, does not mean that all octopuses are crubs, because there could be some octopuses that are not crubs and that is why I though YES was a correct answer, but I could obviously be wrong. My idea that there could be octopuses that are not crubs came from a venn diagram I made to visualize the stimulus (crubs circle contained within the octopuses circle)

I guess its because you cannot assume that information because there is the possibility that All octopuses are curbs. I honestly used to hate these questions because my brain could never grasp what they were trying to ask, especially finding out the difference between an inference and assumption.