The correct answer is C) be lighter. High-expansion foam has a much lower density compared to low-expansion foam, making it lighter. This is because high-expansion foam has a higher air-to-solution ratio, with the foam containing a large volume of air trapped within the foam structure. In contrast, low-expansion foam has a higher liquid content, resulting in a denser and heavier foam. The lighter weight of high-expansion foam makes it more suitable for applications where weight is a critical factor, such as portable fire extinguishing systems. The other options are incorrect because: A) high-expansion foam may not cling as well to vertical surfaces due to its lighter nature, but this is not the primary difference compared to low-expansion foam; B) high-expansion foam is not necessarily more heat resistant than low-expansion foam; and D) high-expansion foam is not inherently "wetter" than low-expansion foam.