The correct response is:
The expressions are equivalent because Ella got the same result when she substituted one for f.
This is because both expressions, when evaluated with \( f = 1 \), resulted in \( 8.6 \). However, this does not necessarily mean they are equivalent for all values of \( f \); the equivalence would need to be proven for all possible values of \( f \). Therefore, strictly speaking about the equivalence based on the given test, her substitution demonstrates they yield the same result for that particular case.