Option 1: Softwood refers to wood that is obtained from coniferous trees, such as pine or spruce. While softwood is generally less dense and less resistant to decay compared to hardwood, it does not necessarily become useless soon after exposing it to open air. Therefore, option 1 is not the correct answer.

Option 2: Fibrous wood refers to wood that has a fibrous or stringy texture, such as bamboo or certain species of palm trees. Like softwood, fibrous wood is typically less dense and less resistant to decay compared to hardwood. However, it does not automatically become useless soon after exposure to open air. Thus, option 2 is not the correct answer.

Option 3: Wet wood is the correct answer. When wood is wet, it becomes susceptible to decay and rot. The moisture content in wet wood creates an environment that promotes the growth of fungi and bacteria, which gradually deteriorate the wood. Wet wood can lose its structural integrity and become useless over time.

Option 4: Hardwood refers to wood that is obtained from broad-leaved trees, such as oak or mahogany. Hardwood is generally denser and more resistant to decay compared to softwood or fibrous wood. Therefore, option 4 is not