Can two streamlines cross each other? Why?

Water flows through a horizontal pipe of variable cross-section at the rate of $12 \pi$ litre per minute. The velocity of the water at the point where the diameter of the pipe becomes $2 \text{ cm}$ is (in $\text{ m/s}$)

$A$ pipe having an internal diameter $D$ is connected to another pipe of the same size. Water flows into the second pipe through $n$ holes, each of diameter $d$. If the water in the first pipe has speed $v$, what is the speed of water leaving the second pipe?

Water enters through end $A$ with speed $v_1$ and leaves through end $B$ with speed $v_2$ of a cylindrical tube $AB$. The tube is always completely filled with water. In case $I$,the tube is horizontal; in case $II$,it is vertical with end $A$ upwards; and in case $III$,it is vertical with end $B$ upwards. We have $v_1 = v_2$ for:

The surface of water in a water tank of cross-section area $750 \text{ cm}^2$ on the top of a house is $h \text{ m}$ above the tap level. The speed of water coming out through the tap of cross-section area $500 \text{ mm}^2$ is $30 \text{ cm/s}$. At that instant, $dh/dt$ is $y \times 10^{-3} \text{ m/s}$. The value of $y$ will be

An incompressible fluid flows steadily through a horizontal cylindrical pipe. The pipe has radius $3R$ at point $A$ and $1.5R$ at point $B$, further along the flow direction at the same level. If the velocity at point $A$ is $v$, then the velocity at point $B$ is: