Identify the compounds A, B, C and D in the given reaction :
`HC-=CH underset(Hg^(2+)//H_(2)SO_(4))overset(H_(2)O)to A underset(K_(2)Cr_(2)O_(7)+H_(2)SO_(4)) overset([O])toB overset(Ca(OH)_(2))toC underset("dry distillation") overset("Heat")toD`

Explanation

To identify the compounds A, B, C, and D in the given reaction, we will analyze each step of the reaction systematically.

### Step 1: Identify Compound A
The reaction starts with an alkyne, specifically ethyne (HC≡CH). When ethyne is treated with water (H2O) in the presence of mercuric ions (Hg²⁺) and sulfuric acid (H₂SO₄), it undergoes hydration to form an aldehyde.

The reaction can be summarized as:
\[ \text{HC≡CH} + \text{H}_2\text{O} \xrightarrow{\text{Hg}^{2+}, \text{H}_2\text{SO}_4} \text{A} \]

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