Linear: refers to the relationship between the quantity and the quantity in a straight line, which represents regular and smooth movement in space and time. According to the above definitions, Which of the following are linear relationships:. ["Teacher Wang's health deteriorates with age.", 'The price of a commodity fluctuates around its value.', 'The altitude of the rocket that takes off in a straight line of uniform acceleration gradually increases with the elapsed time.', 'When Liu took the test, his heart rate was faster then slower before and after the test.']