In which cases is there an obligation to stop before a railway crossing?

1. AWhen a train is seen, heard or sounds a warning signal.
2. BThere is no obligation to stop, but one should use judgement as to whether to stop when a train is seen approaching.
3. CWhen a "give right of way to traffic on the crossing road" sign is set.
4. DWhen a "railway crossing" sign is set.

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