They act like convey belt, transporting water the both sides of the pole
Constant means non-changing, so constant would refer to something that stays the same, does not change.
So the correct answer is this one: the quantity that must remain the same
The measles, mumps and the rubella vaccine [MMR] is usually given to children twice before the age of 7. The first dose is given between age 12 to 15 months while the second dose is given between the ages of 4 and 6. Many people link the MMR vaccines to autism because of a scientific publication that was published in 1989. The research study reported a link between autism and MMR vaccination. This publication has been discredited.
Also, the period of during which the first dose of the vaccination is usually given coincides with the time when parents usually notice developmental abnormalities in their children.
