Commit 162e40

2026-04-28 10:53:45 lisa: Context Diagram: fix CYU Q2 — (b) AND (d) both correct Earlier rationale claimed (d) was false because flows can be bidirectional. That contradicts the curriculum convention (per C022-Lesson-Context-Diagram.md Q5): each data flow is a single- headed arrow, and bidirectional data MUST be drawn as two separate arrows. Reframed as 'select all that apply' so both correct statements stand and the two-arrow convention is taught explicitly.

sd/C02/Context Diagram.md ..
@@ 136,10 136,13 @@
	>! **(d) data store.** Data stores belong on the Level-1 DFD, not the context diagram. At Level 0 the whole system is a single black box — its internal storage is hidden.
-	**2.** In a context diagram, which statement is correct?
+	**2.** In a context diagram, which statements are correct? *(select all that apply)*
	*(a) Entities can connect directly to other entities · (b) Data flows can only connect entities to the system process · (c) Multiple processes can be shown · (d) All flows must be unidirectional*
-	>! **(b) Data flows can only connect entities to the system process.** Every arrow must touch the system circle. Entity↔entity flows are never allowed; (c) describes a DFD, not a context diagram; (d) is false — a flow can be bidirectional if data moves both ways.
+	>! **Both (b) and (d) are correct.**
+	>! **(b)** — Every arrow must touch the system circle. Entity↔entity flows are never allowed.
+	>! **(d)** — Each data flow is a **single-headed (unidirectional) arrow** labelled with the data crossing that way. If data moves *both* ways between an entity and the system, you must draw **two separate arrows** (one each direction), each with its own label — *not* one line with arrowheads at both ends.
+	>! **Why (a) and (c) are wrong:** (a) breaks the entity-to-system rule above; (c) describes a DFD — a context diagram has exactly one process circle (the whole system).
	**3.** You're drawing a context diagram for a Library Management System. How should *"borrowing a book"* appear?
	*(a) An entity labelled "Borrowing" · (b) A process inside the system · (c) A data flow labelled "Book borrowing request" from Borrower to the system · (d) A rectangle showing the book collection*