In the letter series, some of the letters are missing. Complete the given letter series by choosing the correct alternative: b _ b _ bb _ bbb _ bb _ b. Choose the correct answer from the options given below.

Filling the gaps with a, a, b, a, a, b completes the intended pattern.

The series builds blocks of b of growing then shrinking length.

Single letters are inserted between these blocks of b.

The inserted letters follow the repeating run a, a, b, a, a, b.

Placing them in the blanks restores the symmetric design of the series.