If the route between G and C is closed, which one of the following stations need not be passed through while travelling from H to C?
- (a) E
- (b) D
- (c) A ✓ UPSC's answer
- (d) B
Why the answer is (c)
• With G-C closed, from H the only outgoing passage is H to G, so every H-to-C route must start H-G.
• From G the only usable outgoing passage is G to B, because G-C is closed and G-B is two-way, so the route must continue G-B.
• From B the only forward passage toward C is B to F, and from F the only forward passage is F to D, giving H-G-B-F-D.
• From D the only passage that can lead to C is D to E, since D-H and D-F only return to earlier stations.
• From E the direct one-way passage E to C reaches C; E to A is a dead end because A connects only back to E, so A need not be passed.
• Therefore the necessary route is H-G-B-F-D-E-C, and the station that need not be passed is A, option (c).
Why the other options are wrong
- (a) E
- E is mandatory because after reaching D the only passage that can lead to C is D-E and then E-C.
- (b) D
- D is mandatory because after B-F the only passage that can lead to C is F-D.
- (d) B
- B is mandatory because after H-G the only usable passage is G-B.
Asked in the CSAT Paper II of the UPSC Civil Services Preliminary Examination 2017, held on 18 June 2017. Question and answer key: Union Public Service Commission. Explanation: UPSC Answer Check.