Consider the following statements:1. In flow through hydro-dynamically smooth pipes, the friction factor f is always a constant.2. In flow through hydro-dynamically smooth pipes, the friction factor f is always a function of the flow Reynolds number.3. In a fully developed rough turbulent pipe flow, the friction factor f is a function of relative roughness only.4. In a fully developed rough turbulent pipe flow, the friction factor f is a function of the flow Reynolds number and relative roughness.Which of the above statements are correct?

• A. 1 and 3
• B. 2 and 3
• C. 2 and 4
• D. 1 and 4