Phosgene is formed slowly from which one of the following on exposure to air and sunlight?

The collector plate in an experiment on the photoelectric effect is kept vertically above the emitter plate. $A$ light source is turned on and a saturation photocurrent is recorded. An electric field is then switched on which has a vertically downward direction.

In a book of $500$ pages,it is found that there are $250$ typing errors. Assume that Poisson law holds for the number of errors per page. Then,the probability that a random sample of $2$ pages will contain no error,is :

Which of the following is a characteristic of $Pteridophytes$?

The richest sources of vitamin $B_{12}$ are