Among the compounds
$(i)$ $HC \equiv C-COOH$
$(ii)$ $CH_2=CH-COOH$
$(iii)$ $CH_3-CH_2-COOH$ and
$(iv)$ $CH_3-CH_2-OH$
The correct order of acid strength is

Fill in the blanks:
$(1)$ Two isomers of butane are $n$-butane and isobutane. The relation of the number of $\sigma$ and $\pi$-bonds in them is .......... .
$(2)$ .......... is present in the structure of a heterocyclic compound.
$(3)$ Cyclohexene is a .......... type and furan is a .......... type of compound.
$(4)$ .......... type of $3-D$ model indicates the atomic volume but does not indicate the bond.

The enolic form of acetone contains ................

The number of secondary carbon atoms present in the following compounds are respectively:

Enol content is highest in

Glycerol is not used in which of the following cases?