Let $X$ be a set with the property that for any two metrics $d _ { 1 }$, and $d _ { 2 }$ on $X$, the identity map
$$id : \left( X , d _ { 1 } \right) \rightarrow \left( X , d _ { 2 } \right)$$
is continuous. Which of the following are true?\\
(a) $X$ must be a singleton.\\
(b) $X$ can be any finite set.\\
(c) $X$ cannot be infinite.\\
(d) $X$ may be infinite but not uncountable.