Select the combination of letters that when sequentially placed from left to right in the blanks of the given series will complete the letter series.
$$\text{C \_ \_ B A C C \_ \_ A C C D \_ \_ C \_ D \_ A}$$

Let's count the total number of terms including blanks, which is 20. We can split the pattern into 4 groups of 5 letters each:
$$\text{C \_ \_ B A | C C \_ \_ A | C C D \_ \_ | C \_ D \_ A}$$
By comparing the repeating structural positions across groups:The first group: $\text{C C D B A}$ (fills: C, D)The second group: $\text{C C D B A}$ (fills: D, B)The third group: $\text{C C D B A}$ (fills: B, A)The fourth group: $\text{C C D B A}$ (fills: C, B)Putting the filled letters in sequence from left to right gives: C, D, D, B, B, A, C, B.Therefore, the correct option is CDDBBACB.