Boyle's law states that the relationship between pressure and volume of a fixed amount of gas at a fixed temperature is inversely proportional. This means that the higher the pressure, the lower the volume, in a proportional relationship. Therefore, of the statements listed, b: 'the temperature and the number of molecules must remain constant for the law to apply' and d: 'as the pressure of a gas increases, the volume decreases proportionally' are correct.
