In which type of crops is the mass selection method used?

Which method is used to obtain virus-free plants?

If the distance between the points $(a \cos \theta, a \sin \theta)$ and $(a \cos \phi, a \sin \phi)$ is $2a$,then $\theta$ is equal to

The main structural component of a virus is:

Which of the following is not tetrahedral?

Given below are $2$ statements:
Statement $I$: The number of species among $BF_4^-$, $SiF_4$, $XeF_4$ and $SF_4$, that have unequal $E-F$ bond lengths is $2$. Here, $E$ is the central atom.
Statement $II$: Among $O_2^-$, $O_2^{2-}$, $F_2$ and $O_2^+$, $O_2^-$ has the highest bond order.
In the light of the above statements, choose the correct answer from the options given below