Quiz: At Most K Pattern

To count subarrays with exactly $k$ distinct elements, why compute atMost($k$) - atMost($k-1$)?

1. A.It's faster than counting directly
2. B.Counting 'exactly $k$' directly is complex; 'at most' has simpler window logic
3. C.Direct counting requires $O(n^2)$
4. D.It reduces memory usage