Inspiration is the process of breathing in air, and it occurs when the pressure in the lungs is lower than the atmospheric pressure. This is because the air flows from the region of high pressure (atmosphere) to the region of low pressure (lungs). Thus, a negative pressure gradient is needed to pull the air into the lungs during inspiration. The contraction of the diaphragm and external intercostal muscles increase the volume of the thoracic cavity, which in turn reduces the pressure within the lungs, allowing air to rush in. Therefore, the correct option is B.