How many cyclic structures are possible for $C_4H_6$?

How are the following compounds related?

Which of the following is not a Lewis acid?

Which of the following key factors makes a plasmid a suitable vector in genetic engineering?

If $x^2+px+1$ is a factor of the expression $ax^3+bx+c$,then

Two bar magnets $A$ and $B$ are placed one over the other and are allowed to vibrate in a vibration magnetometer. They make $20$ oscillations per minute when the similar poles of $A$ and $B$ are on the same side,while they make $15$ oscillations per minute when their opposite poles lie on the same side. If $M_A$ and $M_B$ are the magnetic moments of $A$ and $B$ and if $M_A > M_B$,the ratio of $M_A$ and $M_B$ is