What is the minimal from of Karnaugh map shown below? (Assume that x denotes a don't care term)

• A. b̅ b̅
• B. b̅ d̅ + b̅ c̅
• C. b̅ d̅ + a̅ d̅ c̅ d̅
• D. b̅ d̅ + b̅ c̅ + c̅ d̅