Which cube is a unit cube?
a. cube that is 1 meter long, 1 meter wide, and 1 meter high
b. cube that is 5 inches long, 5 inches wide, and 5 inches high
c. cube that is 7 feet long, 7 feet tall, and 7 feet high
d. cube that is 15 centimeters long, 15 centimeters wide, and 15 centimeters high