What is $C$ in the following reaction sequence?

The compound obtained by the reduction of propionaldehyde by amalgamated zinc and concentrated $HCl$ is

Self-condensation of two moles of ethyl acetate in the presence of sodium ethoxide yields:

Compound $(A)$ with molecular formula $C_6H_{12}O_3$ gives a positive iodoform test and a negative Tollens' test. Upon hydrolysis with a drop of $H_2SO_4$,it gives a product that shows a positive Tollens' test. Compound $(A)$ is:

The end product in the following sequence of reaction is
$HC \equiv CH$ $\xrightarrow{1\% \, HgSO_4, 20\% \, H_2SO_4} A$ $\xrightarrow{CH_3MgX} B$ $\xrightarrow{[O]}$

When the compound $CH_3COCH_2CH_2CH_2CH_2CHO$ undergoes intramolecular aldol condensation,the major product formed is $:$