Cardiac glycosides are developed by milk weed/tobacco plant to defend from grazers.

Explanation

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks about the role of cardiac glycosides produced by milkweed or tobacco plants in defending against grazers.

2. **Identifying Cardiac Glycosides**: Cardiac glycosides are toxic compounds that can affect the heart function of animals that consume them. They are produced by certain plants as a defense mechanism.

3. **Defensive Mechanism of Plants**: Since plants cannot move away from grazers (herbivores), they have evolved various defense mechanisms. These can be morphological (like thorns and spines) or chemical (like toxic compounds).

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