Consider the following context-free grammar in BNF,
where and are non-terminals and +, -, and id are terminals.
::= +
::= -
::=
::= id
a. (10 p.) Construct parsing steps for the string id + id - id in the grammar and state whether or not the string is a sentence in the language.

b. (5 p.) Draw a parse tree from (a)

c. (10 p.) Prove that the grammar is ambiguous.