Final answer:
The sequence is an arithmetic sequence with a common difference of 15.
Step-by-step explanation:
The sequence given is:
y = 21, 36, 51, 66, 81, 96, 111, 126
To determine if the sequence is a geometric sequence or an arithmetic sequence, we can check for a common difference or a common ratio.
Looking at the differences between consecutive terms, we can see that the common difference is 15, which means the sequence is an arithmetic sequence. Therefore, the correct answer is C. y is an arithmetic sequence.