Identify A and B in the following sequence of reactions:
`CH_(3)CH_(2)Br overset(KCN)toA overset(LiAlH_(4))toB`.

Explanation

To solve the problem, we will identify the compounds A and B in the given sequence of reactions.

### Step 1: Identify Compound A
1. **Starting Material**: The starting material is ethyl bromide (CH₃CH₂Br).
2. **Reagent Used**: The reagent used is potassium cyanide (KCN).
3. **Reaction Mechanism**: KCN dissociates into K⁺ and CN⁻. The CN⁻ acts as a nucleophile and attacks the carbon atom bonded to the bromine in ethyl bromide, resulting in the substitution of the bromine atom.
4. **Product Formation**: The bromine atom is replaced by the cyanide group (CN), leading to the formation of propionitrile (CH₃CH₂CN).

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