1. The problem statement, all variables and given/known data For a flow of a compressible fluid through a narrow tube, the following statement is correct a) the volume of the fluid crossing any section of the tube is constant. b) density of the fluid is independent of pressure. c) mass of the fluid crossing any section of the tube varies with time. d) mass of the fluid crossing any section of the tube per unit time is constant. 3. The attempt at a solution The confusion I'm having here is that if a certain volume is passing through the tube then a certain mass is also associated with it and since it is a compressible fluid I'm assuming a gas (picturing the nozzle of a tyre). Moreover since it is a narrow tube with a compressible gas there is obviously a constant flow, unless the tube (pictured just for easier understanding) is already deflated, then it stops quite abruptly. So I'm guessing a) or d) but unable to understand something basic here. Please help me out. Thanks.