The equation of the pair of lines joining the origin to the points of intersection of $x^2+y^2=9$ and $x+y=3$ is:

The venous system of a frog differs from that of a mammal due to the presence of:

The figure below shows two paths that may be taken by a gas to go from a state $A$ to a state $C$. In process $AB$,$400 \, J$ of heat is added to the system and in process $BC$,$100 \, J$ of heat is added to the system. The heat absorbed by the system in the process $AC$ will be ............. $J$.

Consider the following statements regarding Werner's theory of coordination compounds:
$(1)$ Ligands are connected to the metal ions by coordinate bonds (dative bonds).
$(2)$ Secondary valencies have directional properties.
$(3)$ Secondary valencies are non-ionisable.
Which of these statements are correct?

$10$ cells,each of emf $E$ and internal resistance $r$,are connected in series to a variable external resistance. The figure shows the variation of the terminal potential difference of their combination with the current drawn from the combination. The emf of each cell is ............. $V$.

Nucellar polyembryony is reported in species of