The sum of the first terms of a series is . Which one of the following is correct?
- (a) The terms of S form an arithmetic progression with common difference 14.
- (b) The terms of S form an arithmetic progression with common difference 6.
- (c) The terms of S form a geometric progression with common ratio 10/7.
- (d) The terms of S form a geometric progression with common ratio 11/4.