Professor won't give full credit for correct homework question
The professor may have sensible reasons for counting the alternate answer as inferior.  It could be that on the surface, your answer is strictly correct.  But in the context of the class, such an answer might indicate that you hadn't learned what you were supposed to.  

I often give exam problems for which more than one technique will yield the correct answer.  (This is mathematics.)  But part of what I'm testing is whether the student, who should know both or all three techniques, can chose the best technique for this situation.  Naturally, I have students arguing this with me regularly.  They think that as long as they do a problem the right way, they should get full credit.  My response is that I'm trying to measure their learning and mastery of the material, and their poor choice of technique indicates less than ideal mastery.  

Oversimplified example:  Suppose it's 3rd grade and the question says, "What's 314 times 256?"  If a student adds 314 to itself 256 times, that's a correct method, but it tells me that he hasn't learned how to do 3-digit multiplication.  He won't get full credit from me.  

You might try asking the prof why he thinks your answer is inferior and see what he says.