In the given reactions identify A and B.

1. Identification of Reactant 'A'

• The Reaction: Alkyne reduction over palladium on carbon ($$\text{Pd/C}$$) usually reduces an alkyne completely to an alkane. 
• Analysis: The product has a 5-carbon continuous chain with a double bond at position 2. Therefore, the starting material A must be 2-Pentyne ($$\text{CH}_3-\text{C}\equiv\text{C}-\text{C}_2\text{H}_5$$).

2. Identification of Product 'B' (Birch Reduction)

• Mechanism: This is a classic Birch Reduction of an internal alkyne. It proceeds via a radical-anion mechanism that favors anti-addition of hydrogen atoms due to minimizing steric repulsion between the bulky alkyl groups.
• Product 'B': An internal alkyne reduced with $$\text{Na/liquid NH}_3$$ selectively yields a trans-alkene. Thus, B is trans-2-butene.