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Correct answer: C4 = one-way road: traffic in the SIDE STREET drives in the arrow's direction — a rectangular information sign about that street, not a command for your direction; commands are round (D series).; On narrow single-carriageway roads outside the built-up area: watch out for a sharp road edge and/or lower verge; if your right-hand wheels leave the road, you quickly lose control of the steering.
C3/C4 are the open side of the one-way road: entering is allowed, in the arrow's direction; the closed end carries the round C2
Why learners get this wrong: On a one-way street you may reverse along the carriageway to a turning you missed.