In our coursebook, introducing phonology by David Odden, one of the exercise questions asks us to decide if the obstruents of Thai are phonemes or allophones. My teacher says they are allophones but it is hard to predict what their underlying form is.
He gives the following rule to explain the distribution:
[-sonorant] becomes [-voiced] in the environment of (word-final position).
I am confused, how could they be allophones if there is a minimal pair(rap and rak)?
Are they actually allophones? or are they phonemes? The rule the teacher gives doesn't add up but I couldn't make up another one to convince him.