If a breeder wants to introduce a disease-resistant strain, what should be the first step?

Two masses of $10\,kg$ and $20\,kg$ respectively are connected by a massless spring as shown in the figure. $A$ force of $200\,N$ acts on the $20\,kg$ mass. At the instant when the $10\,kg$ mass has an acceleration of $12\,m/s^2$ towards the right,the acceleration of the $20\,kg$ mass is: (in $,m/s^2$)

In an optical fibre,the core and cladding are made with materials of refractive indices $1.5$ and $1.414$ respectively. To observe total internal reflection,what will be the range of the incident angle with the axis of the optical fibre?

Which of the following is a $C_4$ plant?

On combustion of $Li$,$Na$,and $K$ in excess of air,the major oxides formed,respectively,are:

$A$ uniform metal rod of $2\,mm^2$ cross-section fixed between two walls is heated from $0\,^{\circ}C$ to $20\,^{\circ}C$. The coefficient of linear expansion of the rod is $12 \times 10^{-6}\,/^{\circ}C$. Its Young's modulus of elasticity is $10^{11}\,N/m^2$. The energy stored per unit volume of the rod will be ....... $J/m^3$.