According to the Equation of Continuity, if we have a mass of a steady airflow travelling through a tube of varying cross sectional diameter, the mass of air flowing through each cross section must be the same.

In order to maintain the same mass flow the speed will increase or decrease depending on whether the section is widening (speed decreases) or narrowing (speed increases).

The area in the middle is narrower than the area at the entry and exit of the tube. It is expected that the air mass will be the fastest in the middle section, so B and C must be the last if the letters are arranged in order of increasing velocity, with C being the lastest one where the tube is the narrowest.

The area of entry and exit, A and D, are the widest areas. So, it is expected that the airflow will be the slowest when passing from entry and exit.

Therefore, the letters are arranged from the lowest to the highest velocity as follows: A, D, B, C.