Final answer:
Decreasing the temperature and increasing the volume of a gas cause a decrease in pressure, while increasing the number of gas molecules increases pressure. This is explained by the kinetic theory of gases and Boyle's law.
Step-by-step explanation:
Of the following options, decreasing the temperature and increasing the volume would cause a decrease in the pressure of a sample of gas. Increasing the number of gas molecules typically increases the pressure, provided the volume is constant. The temperature, volume, and amount of gas are all factors that affect gas pressure according to the kinetic-molecular theory of gases.
Physically, here's what is happening: Decreasing the temperature of a gas slows down its molecules, which results in fewer collisions with the container walls, thus reducing pressure. When the volume of a container is increased, the gas molecules have more space to move around, which leads to fewer collisions per unit area, and thus, lower pressure. By contrast, increasing the number of gas molecules in a constant volume increases collisions and therefore increases pressure.
According to Boyle's law, pressure and volume are inversely related. Therefore, if the pressure is decreased, the volume should increase, and vice versa, all else being equal. In summary, a decrease in temperature or an increase in volume will lead to a decrease in gas pressure.