**Core Concept**
The gingival bleeding index (GBI) is a simple, non-invasive method to assess gingival health. It involves measuring the time elapsed before bleeding occurs when a gentle probe is inserted between the teeth and the gingiva. This index is used to monitor gingivitis, an inflammatory condition of the gingiva caused by poor oral hygiene.

**Why the Correct Answer is Right**
The GBI is based on the idea that healthy gingiva does not bleed when probed, but inflamed gingiva does. The Ainamo and Bay index specifically uses a 10-second time frame to assess bleeding. If bleeding occurs within 10 seconds, it indicates inflammation and a positive score is recorded. This short time frame allows for early detection of gingivitis, enabling timely intervention to prevent further progression.

**Why Each Wrong Option is Incorrect**
**Option B:** 20 seconds is too long to be considered a positive score in the Ainamo and Bay index, as it may not accurately reflect the presence of gingivitis.
**Option C:** 30 seconds is even longer and would likely miss many cases of gingivitis.
**Option D:** 40 seconds is far beyond the time frame used in the Ainamo and Bay index, making it an incorrect choice.

**Clinical Pearl / High-Yield Fact**
When assessing gingival health using the GBI, it's essential to use a gentle probing technique to avoid causing artificial bleeding, which can lead to false positive results.

**✓ Correct Answer: A. 10 Sec.**