If two reactants part in a reaction, then:

Options

A. It can never be of second order
B. It can never be of first order
C. It can never be unimolecular
D. It can never be bimolecular

Answer

To solve the question regarding the reaction between two reactants, let's analyze the situation step by step.

### Step 1: Understand the Reaction
We have two reactants, A and B, participating in a reaction represented as:
\[ A + B \rightarrow P \]
This indicates that both reactants are involved in the formation of the product P.

### Step 2: Identify the Order of Reaction
The order of a reaction is determined by the sum of the powers of the concentration terms in the rate law expression.

### Step 3: Write the Rate Law
For the reaction \( A + B \rightarrow P \), the rate law can be expressed as:
\[ \text{Rate} = k[A]^m[B]^n \]
where \( m \) and \( n \) are the orders with respect to reactants A and B, respectively.

### Step 4: Analyze Possible Orders
1. **Zero Order**: This would imply that the rate of reaction does not depend on the concentration of A or B, which is not possible here since both reactants are involved.
2. **First Order**: It is possible for the reaction to be first order with respect to one reactant and zero order with respect to the other, or first order with respect to both.
3. **Second Order**: The reaction can be second order if \( m + n = 2 \). This can happen if both reactants are first order, or one is first order and the other is zero order.
4. **Unimolecular**: A unimolecular reaction involves only one reactant in the rate-determining step, which is not the case here since we have two reactants.

### Step 5: Conclusion
Based on the analysis:
- The reaction cannot be zero order.
- It can be first order.
- It cannot be unimolecular.
- It can be second order.

### Final Answer
Thus, the correct conclusion is that the reaction can be first order or second order but cannot be unimolecular.