Answer:
D. She showed that f(n) - f(n - 1) was a constant difference.
Step-by-step explanation:
∵ A sequence is called arithmetic if there is constant difference between consecutive numbers,
Thus, option D is correct.
In Explicit formulas, we define each term in a sequence directly,
Thus, there are many sequences other than arithmetic that have explicit formula,
i.e. Option A can not be true.
Now, recursive formula defines the relation between successive terms,
Thus, there are many sequences other than arithmetic that have recursive formula,
i.e. Option B can not be true.
Also, if the ratio between successive terms is constant then the sequence is a Geometric sequence,
A GP can not be an AP,
i.e. Option D can not be true.