Stress refers to the response of living things to external stimuli. It is a measure of biological advantage and avoidance of harm. It is also one of the basic characteristics of living things. Which of the following are stressful? ['The firecracker explodes when it ignites.', 'Throwing stones into the lake waves.', 'The hand touches the burning candle and retracts immediately.', 'Iron nails rust in a humid environment.']