The correct response is:
The expressions are equivalent because Ella got the same result when she substituted zero for f.
This is because Ella found that both expressions equal 3.6 when she substituted 0 for \( f \). This indicates that both expressions yield the same result for that particular value of \( f \). However, while they may give the same output for \( f = 0 \), it does not necessarily mean they are equivalent for all values of \( f \) unless further testing is done with other values of \( f \).