If $n(A) = 3$ and $n(B) = 6$ and $A \subseteq B$,then the number of elements in $A \cap B$ is equal to

Based on the compounds of group $15$ elements, the correct statement($s$) is (are)

$(A)$ $Bi _2 O _5$ is more basic than $N _2 O _5$

$(B)$ $NF _3$ is more covalent than $BiF _3$

$(C)$ $PH _3$ boils at lower temperature than $NH _3$

$(D)$ The $N - N$ single bond is stronger than the $P - P$ single bond

Which one of the following is not a condensation polymer?

When a certain metallic surface is illuminated with monochromatic light of wavelength $\lambda$,the stopping potential for photoelectric current is $3V_0$. When the same surface is illuminated with light of wavelength $2\lambda$,the stopping potential is $V_0$. The threshold wavelength of this surface for the photoelectric effect is:

Which one of the following statements about packing in solids is incorrect?

In an emission spectrum,the transitions $n_2 \to n_1, n_3 \to n_1, n_4 \to n_1, n_5 \to n_1$ result in the emission of energies $E_1, E_2, E_3, E_4$ respectively. What is the increasing order of these energies?