When, in separate reactions, 2-methylpropene, propene, and ethene are allowed to react with HI under the same

Question:

When, in separate reactions, 2-methylpropene, propene, and ethene are allowed to react with HI under the same conditions (i.e., identical concentration and temperature), 2-methylpropene is found to react fastest and ethene slowest. Provide an explanation for these relative rates.