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Assertion (A): The sum of the first fifteen terms of the AP $21, 18, 15, 12, \dots$ is zero.
Reason (R) : The sum of the first $n$ terms of an AP with first term '$a$' and common difference '$d$' is given by $S_n = \frac{n}{2} [a + (n - 1) d]$.
Reason (R) : The sum of the first $n$ terms of an AP with first term '$a$' and common difference '$d$' is given by $S_n = \frac{n}{2} [a + (n - 1) d]$.
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(C) Assertion (A) is true, but Reason (R) is false.