Select the combination of letters that when sequentially placed in the blanks of the given series will complete the series. _bc_efa_cd_fa_c_ef_bc_ef

abbebdad adbebbad adbebdad addebdad

adbebdad

In this series _bc_efa_cd_fa_c_ef_bc_ef  adbebdad  is the repeated word The complete series is: abcdef / abcdef / abcdef / abcdef   The letter cluster required is adbebdad