The herbicide DCMU kills the weeds because it inhibits

Options

A. respiration
B. `CO_(2)` fixation
C. cell division
D. `NO_(3)^(2-)` uptake

Explanation

To answer the question "The herbicide DCMU kills the weeds because it inhibits," we will follow these steps:

### Step 1: Understand what DCMU is
DCMU stands for dichlorophenyldimethylurea. It is a herbicide that is known to affect plant processes.

**Hint:** Remember the full form of DCMU and its classification as a herbicide.

### Step 2: Identify the primary function of DCMU
DCMU primarily inhibits photosynthesis in plants. It specifically disrupts the light-dependent reactions of photosynthesis.

**Hint:** Focus on the role of DCMU in photosynthesis and its impact on light reactions.

### Step 3: Determine the specific target of DCMU
DCMU blocks the electron transport chain by inhibiting Photosystem II. This blockage prevents the transfer of electrons to plastoquinone, which is crucial for ATP synthesis.

**Hint:** Recall the components of the light-dependent reactions and how they relate to ATP production.

### Step 4: Understand the consequences of inhibiting Photosystem II
Since ATP synthesis is blocked due to the inhibition of Photosystem II, the energy required for the dark reactions (Calvin cycle) is not produced. The dark reactions depend on ATP and NADPH generated in the light reactions.

**Hint:** Consider how the light reactions and dark reactions are interconnected in photosynthesis.

### Step 5: Conclude the impact on carbon dioxide fixation
Without ATP and NADPH, the dark reactions cannot occur, leading to a halt in carbon dioxide fixation. This means that the plant cannot produce glucose, which is essential for its growth and survival.

**Hint:** Think about the overall process of photosynthesis and the importance of carbon dioxide fixation.

### Final Answer
The correct answer is **carbon dioxide fixation** (Option 2) because DCMU inhibits the light reactions of photosynthesis, which are necessary for the dark reactions to occur.

### Summary of Steps:
1. Identify DCMU as a herbicide.
2. Recognize its role in inhibiting photosynthesis.
3. Understand its specific action on Photosystem II.
4. Realize the consequences on ATP synthesis and dark reactions.
5. Conclude that carbon dioxide fixation is inhibited.