What is the order of the recurrence an=an−1+an−2+⋯+an−ka_n = a_{n-1} + a_{n-2} + \dots + a_{n-k}an=an−1+an−2+⋯+an−k?
k−1k-1k−1
kkk
k+1k+1k+1
2k2k2k