A discrete-time, Linear time-invariant system with input sequence xn and output sequence yn is characterized by:yn = 0.1 xn + 0.9 yn-1If two such systems are connected in series, which of the following is the governing differential equation of the overall system?

• A. yn – 1.8 yn-1 + 0.81 yn-2 = 0.01 xn
• B. yn + 0.81 yn-1 = 0.01 xn
• C. yn – 0.81 yn-1 + 1.8 yn-2 = 0.01 xn
• D. yn – 1.8 yn-I = 0.01 xn