Which of the following are expected not to have zero entropy even at absolute zero ?
`NaCl, NO , CO_2, CO `

Explanation

To determine which substances are expected not to have zero entropy even at absolute zero, we need to consider the nature of the substances listed: NaCl, NO, CO2, and CO.

### Step-by-Step Solution:

1. **Understanding Entropy at Absolute Zero**:
According to the third law of thermodynamics, the entropy of a perfect crystalline substance approaches zero as the temperature approaches absolute zero (0 K). However, this applies only to substances that have a perfect order at absolute zero.

2. **Analyzing Each Substance**:
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