C -> A | b c
A -> d | b e
If a grammar can generate the same sentence in multiple different ways (with different parse tress) then it is ambiguous. An ambiguity must start with a context clash (but not all context clashes imply ambiguity). To see if a context clash is also a case of ambiguity you would need to follow the alternatives involved in each context clash to see if they can generate the same complete sequence of tokens.