The electron affinity values of elements $A, B, C$ and $D$ are respectively $-135, -60, -200$ and $-348 \ kJ \ mol^{-1}$. The outer electronic configuration of element $B$ is

The principle of vaccination is based on which property of the immune system?

If $\cos ^{-1}\left(\frac{x^2-y^2}{x^2+y^2}\right)=k$ (a constant),then $\frac{d y}{d x}$ is equal to

When radiation of wavelength $\lambda$ is incident on a metallic surface,the stopping potential is $4.8 \ V$. If the same surface is illuminated with radiation of double the wavelength,then the stopping potential becomes $1.6 \ V$. Then the threshold wavelength for the surface is

The transformed equation of $3x^2 + 3y^2 + 2xy = 2$,when the coordinate axes are rotated through an angle of $45^{\circ}$,is

What type of relationship is observed between insect-pollinated flowers and their pollinating insects?