$\frac{a}{v}$
$av$
According to the equation of continuity for a flowing incompressible fluid, the mass flow rate remains constant throughout the tube. The equation is expressed as:
\[ A_1 v_1 = A_2 v_2 \] Where: - \( A \) is the cross-sectional area of the tube, - \( v \) is the velocity of the fluid. This means that the product of the cross-sectional area \( a \) and velocity \( v \) at any point in the tube remains constant. Thus, the quantity that remains constant is: \[ a \cdot v \] Therefore, the correct answer is that the product of the cross-sectional area and velocity remains constant.
Correct Answer: (D) \( a \cdot v \)