In broadcasting radios, there are two modes of receiving signals, AM and FM. In which mode, the signals will be smooth while the broadcasting radio is crossing an underground passage.

Explanation

To determine which mode of receiving signals (AM or FM) will provide smoother signals while a broadcasting radio is crossing an underground passage, we need to analyze the properties of both amplitude modulation (AM) and frequency modulation (FM).

### Step-by-Step Solution:

1. **Understanding AM and FM**:
- AM stands for Amplitude Modulation, where the amplitude of the carrier wave is varied in accordance with the information signal.
- FM stands for Frequency Modulation, where the frequency of the carrier wave is varied based on the information signal.

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