For an arithmetic progression with first term a and common difference d, the sum written forward is added term-wise to the same sum written backward. Which of the following represents 2Sn obtained after this addition?
a) n[2a+(n+1)d]
b) n[2a+(n−1)d]
c) 2n[2a+(n−1)d]
d) n[a+(n−1)d]