my wife the teacher said "how big is the bottle?" she said the question was too vague and therefore could be correct with either answer. she would give your son extra points if he could explain why he was correct (what information is lacking in the question or weights compared).
she said your son should make it a point to go back to the teacher and explain why he choose that answer and to do it with a smile.
in thinking about it, the teacher was actually right, why are you carrying that water, you shouldn't cross any springs or streams with a full water bottle. your bottle should weigh about an ounce, your sleeping bag is at least twice as heavy as that! what are you teaching your son??? ;)