Which on set of letter complete the given series ?
_bc_ca_aba_c_ca

Fill in the blank by suitable letters.
a_c_ba_ca_cb

A series is given with one term missing. Choose the correct alternative from the given ones that will complete the series.
K6V, M12U, O18T,?

Complete the following alphabet series.
ZA : YB :: YC :?

Directions : In each of the following letter series, some of the letters are missing which are given in that order as one of the alternatives below it.

ab ---b -bbaa- 

Complete the following alphabet series.
ABC, PQR, DEF, STU, ?

From the given alternative words, select the word which cannot be formed using the letters of the given word:
MERCHANDISE

How many pair of letters are there in the word “EFFECTIVE” which has as many letters (in both forward and backward direction) in between as they have in English alphabet series?