Let aₙ be the number of n-bit strings that do not contain two consecutive 1s. Which one of the following is the recurrence relation for aₙ?
A. aₙ = aₙ₋₁ + aₙ₋₂
B. aₙ = aₙ₋₁ + 2aₙ₋₂
C. aₙ = 2aₙ₋₁ + aₙ₋₂
D. aₙ = 2aₙ₋₁ + 2aₙ₋₂