What is the minimal form of the Karnaugh map shown below? Assume that $X$ denotes a don’t care term
- $\bar{b} \bar{d}$
- $ \bar { b } \bar { d } + \bar{b} \bar{c} $
- $ \bar{b} \bar{d} + {a} \bar{b} \bar{c} {d}$
- $ \bar{b} \bar{d} + \bar{b} \bar{c} + \bar{c} \bar{d} $