Users complain a RAG-powered product gives stale answers despite documents being updated weekly. The retrieval layer uses dense embeddings refreshed hourly. The LLM is Claude Sonnet 4.6. Which of the following is the MOST LIKELY cause? (a) Embedding model is too small (b) Embedding cache is keyed by document_id and is not invalidated when document content changes (c) Vector index dimensionality is too low (d) The LLM's knowledge cutoff predates the document update (e) Need to add a re-ranking layer