Directions for the following 3 (three) items: Consider the given information and answer the three items that follow.
Eight railway stations A, B, C, D, E, F, G and H are connected either by two-way passages or one-way passages. One-way passages are from C to A, E to G, B to F, D to H, G to C, E to C and H to G. Two-way passages are between A and E, G and B, F and D, and E and D.
While travelling from C to H, which one of the following stations must be passed through?
Answer & explanation
Answer: (b) E
UPSC's official answer: (b) · the answer UPSC accepted, and the one that counts in the exam
Also defensible: (c)
- Exits from each station: A → E; B → F, G; C → A; D → H, F, E; E → G, C, A, D; F → D; G → C, B; H → G.
- Working forward: C's only exit is A, and A's only exit is E, so every route from C passes through E. That is UPSC's (b).
- Working backward: the only passage into H is D → H, so every route must also pass through D. The two complete routes are C → A → E → D → H and C → A → E → G → B → F → D → H; both contain E and D.
- G and F lie only on the longer route, so (a) and (d) are wrong; but (b) and (c) are both forced, and the item has two correct options.
UPSC's key is (b) E, which every route must pass through; D is equally unavoidable, so (c) is also correct. In the exam, trace the forced moves from the start station first, as the key does, and give E.
This box is Minimalist IAS's analysis, with its sources; it does not change UPSC's answer.
From C the only exit is to A, and from A the only onward move is to E, so every journey from C to H has to pass through E. G and F lie only on the longer of the two routes, so they are not compulsory.
- Exits from each station: A → E; B → F, G; C → A; D → H, F, E; E → G, C, A, D; F → D; G → C, B; H → G.
- From C the only move is to A, and from A the only move is to E — so E is on every route out of C.
- Routes to H: C → A → E → D → H, or C → A → E → G → B → F → D → H; both pass through E.
- G and F appear only on the longer route, so they are not compulsory; E is the station UPSC's key accepts.
Remember · For 'must pass through', trace the forced moves from the start: a station with a single exit forces the next station onto every route.
Question and answer: UPSC's official GS Paper II (2017, Series A) — paper ↗ · answer key ↗. Explanation: Minimalist IAS, checked 1 Oct 2026 (how we verify). ·