Conisder the following proposed mechanism
`A underset(k_(2))overset(k_(1))hArr B` , `B+C overset(k_(3))rarrD`
For the overall reaction `A+CrarrD`, assuming `B` to be an intermediate described by the steady state approximation, write the rate expresison for `[A]`.

Explanation

To solve the given problem, we will follow the steps outlined in the video transcript while providing a structured approach.

### Step-by-Step Solution:

1. **Identify the Mechanism**:
The proposed mechanism consists of two elementary steps:
- Step 1: \( A \xrightleftharpoons[k_2]{k_1} B \)
- Step 2: \( B + C \xrightarrow{k_3} D \)
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