To double the flow rate, you must increase the pressure:

• A. Fourfold
• B. Twofold
• C. Threefold
• D. Fivefold

Answer: (A)

To double the flow rate in a fluid system, you must increase the pressure fourfold due to the principles outlined in fluid dynamics, particularly applying Bernoulli's equation or the Hagen-Poiseuille equation for laminar flow.

In laminar flow, the relationship between flow rate (Q), pressure (P), and resistance (R) can be expressed by the equation Q = ΔP/R. Here, if the resistance remains constant and you wish to increase the flow rate to twice its original value, the required increase in pressure is not linear. Instead, flow rate is proportional to the fourth power of the radius in a pipe, or in a broader sense, the square root of the pressure differential when considering resistance factors, which means that a quadrupling of pressure is necessary to achieve a doubling of flow rate.

Therefore, to achieve a twofold increase in flow rate, the pressure must be increased fourfold to overcome the resistance and enhance the flow dramatically. This showcases the non-linear relationship between pressure and flow rate in fluid dynamics.