Descriptions of some reactions are given below. Arrange them in the order of chemical displacement, decomposition, double displacement and chemical combination.
(A) Formation of precipitate.
(B) Liberation of `H_2` gas when metal reacts with acid.
(C) Formation of ammonia from nitrogen and hydrogen.
(D) Liberation of oxygen by heating of potassium nitrate.

Options

A. BADC
B. CDAB
C. CADB
D. BDAC

Explanation

To arrange the given reactions in the order of chemical displacement, decomposition, double displacement, and chemical combination, we need to identify each reaction type based on the descriptions provided.

### Step-by-Step Solution:

1. **Identify the Reaction Types**:
- (A) **Formation of precipitate**: This is a characteristic of double displacement reactions where two solutions react to form an insoluble product (precipitate).
- (B) **Liberation of H₂ gas when metal reacts with acid**: This is a chemical displacement reaction where a more reactive metal displaces hydrogen from the acid.
- (C) **Formation of ammonia from nitrogen and hydrogen**: This is a chemical combination reaction where two or more reactants combine to form a single product (ammonia).
- (D) **Liberation of oxygen by heating of potassium nitrate**: This is a decomposition reaction where a single compound breaks down into two or more products.

2. **Arrange the Reactions**:
- **Chemical Displacement**: (B) Liberation of H₂ gas when metal reacts with acid.
- **Double Displacement**: (A) Formation of precipitate.
- **Decomposition**: (D) Liberation of oxygen by heating potassium nitrate.
- **Chemical Combination**: (C) Formation of ammonia from nitrogen and hydrogen.

3. **Final Order**:
- The order of reactions is: **B, A, D, C**.

### Summary of the Order:
1. **Chemical Displacement**: B
2. **Double Displacement**: A
3. **Decomposition**: D
4. **Chemical Combination**: C