Choose the best definitions of parameter and statistic. a. a statistic is a number that describes a population and a parameter is a number that describes a sample. b. a parameter is an unknown characteristic of a group and a statistic is a known characteristic of a group. c. a parameter is a number that describes a population. d. a statistic is a number that is used to estimate a parameter. e. a statistic is a variable that describes a sample and a parameter is a variable that describes a population. f. a statistic is a number that describes a sample and a parameter is a number that describes a population.