What is meant by the reference body when they say: a) is the truck moving at 70 km / h?

What is meant by the reference body when they say:
a) is the truck moving at 70 km / h?
b) is an airplane stewardess walking through the cabin at a speed of 1 m / s?
c) the speed of the moon is 1 km / s?

a) the surface of the Earth;
b) the aircraft cabin;
c) the center of the Earth.

Remember: The process of learning a person lasts a lifetime. The value of the same knowledge for different people may be different, it is determined by their individual characteristics and needs. Therefore, knowledge is always needed at any age and position.