When a reaction is progressing.

Options

A. The rate of the reaction goes on increasing
B. The concentration of the products goes on decreasing
C. The concentration of the reactants goes on decreasing
D. The reaction rate always remains constant

Answer

To solve the question about the progression of a chemical reaction, let's analyze the situation step by step.

### Step-by-Step Solution:

1. **Understanding the Reaction**:
- Let's consider a hypothetical reaction where reactants A and B are converting into products C and D.
- The general form can be represented as:
\[ A + B \rightarrow C + D \]

2. **Observation of Concentrations**:
- Initially, we have certain concentrations of reactants A and B.
- As the reaction progresses, the concentrations of the reactants will change, and products will start to form.

3. **Change in Concentration**:
- As the reaction proceeds, the concentration of reactants A and B will decrease because they are being consumed to form products C and D.
- Conversely, the concentration of products C and D will increase as they are formed from the reactants.

4. **Rate of Reaction**:
- The rate of a reaction is generally dependent on the concentration of the reactants.
- According to the rate law, the rate of reaction can be expressed as:
\[ \text{Rate} = k [A]^m [B]^n \]
- Here, \(k\) is the rate constant, and \(m\) and \(n\) are the orders of the reaction with respect to reactants A and B.

5. **Evaluating the Options**:
- **Option 1**: The rate of the reaction goes on increasing.
- This is incorrect because as the concentration of reactants decreases, the rate of reaction will also decrease.
- **Option 2**: The concentration of the product goes on decreasing.
- This is incorrect because the concentration of products increases as the reaction progresses.
- **Option 3**: The concentration of reactants goes on decreasing.
- This is correct because the reactants are being consumed in the reaction.
- **Option 4**: The reaction rate always remains constant.
- This is incorrect because the rate of reaction changes with the concentration of reactants.

6. **Conclusion**:
- The only correct statement is that the concentration of reactants goes on decreasing as the reaction progresses.

### Final Answer:
The correct option is that the concentration of reactants goes on decreasing.