Select the number from among the given options that can replace the question mark (?) in the following series.
21, 23, 20, 22, 19, 21, ?
21+2 =Â 23
23-3 =Â 20
20+2 = 22
22-3 = 19
19+2 = 21
21-3 = 18
Here 18 will come in place of the question mark.
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