What rule can be used to find the next term of the arithmetic sequence?39, 60, 81, 102, 123, . . .

Find the difference between consecutive terms: 60 − 39 = 21, and 81 − 60 = 21. The same difference continues, so add 21 to the previous term each time. The rule is “an = an–1 + 21,” which is choice C.

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