Concept:The choice of a gas collection method depends on the physical properties of the gas.Explanation:A gas is collected by downward displacement of air, upward displacement of air, or over water based on its properties.The two main properties considered are its solubility in water and its density relative to air.Soluble gases cannot be collected over water.Gases lighter or heavier than air are collected by upward or downward displacement of air respectively.Therefore, the method used depends on solubility and density.Answer:A. i and ii only