Which pair is different from the others ?

Options

A. `Na-K`
B. `Ca-Mg`
C. `Li-Mg`
D. `B-Al`

Explanation

To determine which pair is different from the others, we need to analyze the groups to which each element in the pairs belongs. Here’s a step-by-step breakdown of the solution:

### Step 1: Identify the Groups of Each Element
- **Sodium (Na)** and **Potassium (K)**: Both belong to Group 1 (Alkali metals).
- **Calcium (Ca)** and **Magnesium (Mg)**: Both belong to Group 2 (Alkaline earth metals).
- **Lithium (Li)** and **Magnesium (Mg)**: Lithium belongs to Group 1 (Alkali metal) while Magnesium belongs to Group 2 (Alkaline earth metal).
- **Boron (B)** and **Aluminum (Al)**: Both belong to Group 13 (Boron group).

### Step 2: Compare the Pairs
- **Sodium-Potassium**: Same group (Group 1).
- **Calcium-Magnesium**: Same group (Group 2).
- **Lithium-Magnesium**: Different groups (Group 1 and Group 2).
- **Boron-Aluminum**: Same group (Group 13).

### Step 3: Identify the Odd Pair
From the comparison:
- Sodium-Potassium, Calcium-Magnesium, and Boron-Aluminum are all pairs of elements that belong to the same group.
- Lithium-Magnesium is the only pair where the elements belong to different groups.

### Conclusion
The pair that is different from the others is **Lithium-Magnesium**. Therefore, the answer is option 3.

### Final Answer
The different pair is **Lithium-Magnesium** (Option 3).

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