Select the combination of letters that when sequentially placed in the blanks of the given series, will complete the series.
$$\text{yzx\_y\_xxyz\_x\_zxx}$$

The series contains 16 letters in total. Let's try dividing it into blocks of 4 letters:
$$\text{yzx\_ | y\_xx | yz\_x | \_zxx}$$
By matching corresponding positions across the blocks, we can see a repeating structural sequence:Block 1: $\text{y z x x}$ (blank filled with x)Block 2: $\text{y z x x}$ (blank filled with z)Block 3: $\text{y z x x}$ (blank filled with x)Block 4: $\text{y z x x}$ (blank filled with y)The pattern is a single repeating block of $\text{yzxx}$.The letters filled sequentially in blanks are: x, z, x, y.