Arrange the following in decreasing order of acidic nature of

Explanation

(a) `(i) gt (iii) gt (ii)`
(b) ` (ii) gt (i) gt (iii) gt (iv)`
`- NO_(2)` is stabilizing by strong electron withdrawing nature `- CH_(3)` is destabilizing by electron donating nature (+ I) and `- OCH_(3)` is also dstabilizing due to electron donting nature (+ R ,-1)