In the given reactions identify A and B .

In palladium-catalyzed hydrogenations of alkynes, the Lindlar catalyst facilitates the addition of hydrogen to the alkyne to afford the corresponding alkene. When this catalyst is used, the addition of the hydrogen substituent always proceeds via syn addition (addition of the substituent on the same side of the bond).

But$-2-$yne when treated with sodium in liquid ammonia gives trans$-2-$butene. The reaction of alkyne with sodium in liquid ammonia produces sodamide in the process converting alkynes to alkenes. Mechanism of Birch Reduction: -Sodium in liquid ammonia produces solvated electrons.