Niche overlap indicates

Which of the following statements is incorrect for $Nostoc$ and $Anabaena$?

$\frac{1}{2 \cdot 3} + \frac{1}{4 \cdot 5} + \frac{1}{6 \cdot 7} + \frac{1}{8 \cdot 9} + \dots$ is equal to

When ethanol is reacted with $PCl_5$,it gives $A$,$POCl_3$,and $HCl$. The reaction of $A$ with silver nitrite $(AgNO_2)$ gives the major product $(B)$ and $AgCl$. What are $A$ and $B$ respectively?

Which of the following structures contains its own $DNA$?

Which element among the alkaline earth metals primarily forms covalent compounds?