Let the language $D$ be defined in the binary alphabet $\{0,1\}$ as follows:
$D:= \{ w \in \{0,1\}^* \mid \text{ substrings 01 and 10 occur an equal number of times in w} \}$
For example , $101 \in D$ while $1010 \notin D$. Which of the following must be TRUE of the language $D$ ?
- $D$ is regular
- $D$ is context-free but not regular
- $D$ is decidable but not context-free
- $D$ is decidable but not in NP
- $D$ is undecidable