Select the number from among the given options that can replace the question mark (?) in the following series.
503, 509, 521, 539, ?
503, 509, 521, 539, ? in this series all are the multiple of 6
$$509\ -\ 503=6$$
$$521-509=12$$
$$539-521=18$$
so with same logic $$563-539=24$$
so the correct answer is optionC
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