Which of the following is used in optical fibres?

Which of the following statements is incorrect regarding living organisms?

Through which cell of the embryo sac does the pollen tube enter the embryo sac?

$A$ liquid is flowing in a horizontal uniform capillary tube under a constant pressure difference $P$. The value of pressure for which the rate of flow of the liquid is doubled when the radius and length both are doubled is

The figure shows wave fronts in still water,moving in the direction of the arrow towards the interface $PQ$ between a shallow region and a deep (denser) region. Which of the lines shown may represent one of the wave fronts in the deep region?

Identify the logic operation carried out by the following circuit.