**Core Concept**
The PQ interval, also known as the PR interval, is the time from the beginning of the P wave to the start of the QRS complex in an electrocardiogram (ECG). It represents the time taken for the electrical impulse to travel from the atria to the ventricles. The PQ interval is crucial in diagnosing various cardiac conditions, such as first-degree atrioventricular (AV) block.

**Why the Correct Answer is Right**
The normal PQ interval ranges from 120 to 200 milliseconds (ms) in adults. This duration allows for the proper conduction of the electrical impulse from the atria to the ventricles. The PQ interval is influenced by the rate of atrial depolarization, the conduction velocity through the AV node, and the His-Purkinje system. A prolonged PQ interval can indicate first-degree AV block, while a shortened PQ interval may suggest Wolff-Parkinson-White syndrome.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the PQ interval is not measured in seconds; it is measured in milliseconds.
**Option B:** This option is incorrect because the normal PQ interval range does not include values below 120 milliseconds.
**Option C:** This option is incorrect because the PQ interval is not measured in relation to the QRS complex duration; it is a separate interval.

**Clinical Pearl / High-Yield Fact**
When interpreting the PQ interval, it's essential to consider the overall ECG rhythm and the patient's clinical context. A prolonged PQ interval can be a sign of first-degree AV block, which may not cause symptoms but can increase the risk of developing higher-degree AV blocks.

**Correct Answer: B. 120-200 ms**