Which of the following tube lengths produced the greatest flow rate?

• A. 10 mm
• B. 5 mm
• C. 15 mm
• D. 20 mm

Answer: (A)

The main concept is how driving pressure and resistance determine flow through a tube. Flow rate rises when resistance is lower, but real tubes involve a balance: if a tube is too short, the flow may not become fully developed and entrance losses can waste part of the pressure drop; if a tube is too long, friction losses increase and slow the flow. In this setup, the length that best balances these effects is the one that yields the highest flow: long enough to develop a smooth flow, but not so long that friction dominates. That middle-length tube therefore shows the greatest flow, while the shortest has noticeable entrance losses and the longest has higher friction, leading to a reduced flow.