Explanation

A is correct because steam's ability to penetrate the skin causes deeper tissue damage. B is incorrect because while steam can be hot, the primary difference in severity relates to penetration and energy transfer mechanism. C is incorrect because while related, 'penetration' (Option A) is cited as the more direct physical reason for increased severity over boiling water. D is incorrect because pressure alone does not account for the full mechanism of injury compared to steam's penetrating nature.