Which of the following statements about hydrogen are correct? a. Hydrogen is rot free in nature and must be extracted from the bound stat in other substances, such as water or fossil fuels. b. Hydrogen can be stored most commonly as compressed gas in underground caverns. c. Hydrogen has the highest energy density per volume in uncompressed state of all substances. d. No additional energy is necessary to extract hydrogen from water and burn it in internal combustion engine. e. All of these.