True/false/uncertain 1. The slope of the income expansion path is never negative. 2. If there are only two goods, both cannot be superior at the same time. 3. Different levels of income cannot result in the exact same demand curve for a good. 4. A good cannot be both superior and neutral. 5. In theory, the demand curve cannot have a positive slope. 6. If there are only two goods and one is inferior, the other doesn't need to be superior.