In questions pick out the odd thing among the given items.
21-13 = 8
13-9 = 4
9-? = 2
? should be 7 i.e. the first number at the start of fourth term
Similarly
23-15 = 8
15-11 = 4
11-? = 2
? should be 9 i.e. the term at the center of fourth term
hence, fourth term in question is odd one out
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