What is the unit of measurement that defines the space available in a rack? How tall are standard racks?

Will you chech and finish these for me, because i am stumped with them.

What type of nuclear decay causes the atomic number of an element to increase by 1?

Which statement is true if the refractive index of medium a is greater than that of medium b? a. total internal reflection is possible when light travels from air to medium b to medium a. b. total internal reflection is possible when light travels from medium a to medium b. c. total internal reflection is possible when light travels from medium b to medium a. d. total internal reflection is possible when light travels from air to medium a. e. total internal reflection is possible when light travels from air to medium b.