For a fully developed turbulent flow of an incompressible Newtonian fluid through a pipe of constant diameter, which of the following statements is/are $\text{CORRECT?}$
- Reynolds stress, averaged over a sufficiently long time, is zero everywhere inside the pipe.
- Reynolds stress at the pipe wall is zero.
- Average velocity of the fluid is half of its center-line velocity.
- Average pressure gradient in the flow direction is constant.