Which is an arithmetic sequence?

Question

A. 1, 4, 9, 16, . . .

B. 2, 1, –1, –4, . . .

C. 4, 8, 16, 32, . . .

D. 21, 9, –3, –15, . . .

.
help please

Answer 1

an arithmetic sequence has a constant difference between terms. Looking at the differences, we see:

A. 3,5,7, . . .
B. -1,-2,-3, . . .
C. 4,8,16, . . .
D. -12,-12,-12, . . .

I guess the answer is D, eh?