When the pressure falls within the magma, the bubbles of gas dissolved in the magma get bigger. This is because the decrease in pressure reduces the solubility of gases in the magma, causing them to come out of solution and form bubbles. This process is called exsolution. As the bubbles rise to the surface, they can cause the magma to erupt explosively. Therefore, the correct answer is B. The silica in the magma getting hotter, the magma cooling and solidifying, and the density of the magma increasing are not directly related to the decrease in pressure within the magma.