Select the choice that is not a necessary prerequisite to solving addition and subtraction problems at the symbolic level. a. the ability to connect groups and symbols b. the ability to write symbols 0 to 10 c. the ability to do concrete addition and subtraction problems d. the ability to identify numerals 0 to 9

Which statement is correct? the speed of the light ray is the same in air and medium a. the speed of the light ray is the same in air and medium b. medium a and medium b have different densities because light refracts at different angles. medium a and medium b have the same density because the light ray strikes both at the same angle.