31. Select suitable number to be placed in the blank space on the basis of principle of selection of number in other similar two.8 12 2810 8 2114 11 ?(a) 29 (b) 31 (c) 33 (d) 36[Half of 1st Column + double of 2nd column=3rd column number. As 4+24=28, 5+16=21. ...