# Water is flowing through a long horizontal tube.Let $P_A$ & $P_B$ be the pressures at two horizontal points A and B of the tube.

$\begin{array}{1 1}(A)\;P_A\text{ must be equal to }P_B\\(B)\;P_A\text{ must be greater than }P_B\\(C)\;P_A\text{ must be smaller than }P_B\\(D)\;P_A=P_B\text{ only if the cross sectional area of A and B are equal}\end{array}$

If cross sectional area at A and B are not equal then velocities are also not equal.So applying Bernoulli’s equation we will not get equal pressure at $A$ & $B$.
Hence (D) is the correct answer.