The speed of light in a material is 0.50 c. what is the critical angle of a light ray at the interface between the material and a vacuum?

a.   mass number from atomic number

the skeletons of most mammals reach a certain size and then stop growing. however, many animals, including some mammals, keep growing throughout their lives. kangaroos, for example, just keep growing and growing until they die. also, the longer an animal lives the greater the chance it might catch some deadly disease.