Which of the following functional groups is present in aldehyde compounds?

Write the name of the next homologue of $CH_{3}OH$ and $HCOOH$.

How would you name the following compounds?
$(i)$ $CH_{3}-CH_{2}-CH=CH_{2}$
$(ii)$ $CH_{3}-CH_{2}-CH_{2}-C(CH_{3})_{2}-CH_{3}$
$(iii)$ $CH_{3}-CH_{2}-CHO$
$(iv)$ $CH_{3}-CH(OH)-CH_{3}$

Alkynes may be represented by the general formula:

The formula of citric acid is shown below:
$\begin{matrix}
\,\,\,\,\,COOH \\
|\,\,\,\,\,\,\, \\
CH_{2}\, \\
|\,\,\,\,\, \\
HO-C-COOH \\
|\,\,\,\,\, \\
\,CH_{2} \\
|\,\,\,\,\, \\
\,\,\,\,\,\,COOH \\
\end{matrix}$
State the name of the $-COOH$ functional group present in citric acid.

What are the two properties of carbon which lead to the huge number of carbon compounds we see around us?