Concept:Radioactive decay is a spontaneous nuclear process that is independent of external physical conditions.Explanation:Statement I is correct because a radioactive substance emits radiation continuously and spontaneously without any external trigger.Statement II is incorrect because temperature and pressure do not affect radioactive emissions.Radioactive decay depends only on the nature of the nucleus, not on environmental conditions.Statement III is correct because the radiation emitted can penetrate opaque matter, though the penetrating power varies with the type of radiation.Therefore, only statements I and III are correct.Answer:C. I and III only