Four of the following five are alike in a certain way and so form a group. Which is the one that does not belong to that group?

Which of the following groups of alphabets should replace the blank spaces so that the group of alphabets, given in bold, follow a logical pattern from the preceding and the following group of alphabets?

$ab\_ y abc\_xw abcdev\_$

What will come in place of the question mark (?) in the following series?

$AC BE DH ? KQ$

What should come next in the following letter series based on English alphabet?
CEA IKG OQM ?