**Core Concept**
A point source epidemic occurs when a group of people are exposed to a contagious agent from a single source at a single point in time. This type of epidemic is characterized by a rapid increase in cases, as the entire group is exposed simultaneously.

**Why the Correct Answer is Right**
The correct answer is **B. ac**. A point source epidemic tends to **continue over one incubation period** because the exposed individuals are all infected simultaneously, and their symptoms will appear within the same incubation period. This results in a **rapid increase in cases** or an **explosive outbreak**, as the entire group becomes sick at the same time. The epidemic curve will have a single peak, followed by a decline as the cases resolve.

**Why Each Wrong Option is Incorrect**
* **Option A:** "Continue over one incubation period" is partially correct, but the other option "Produce multiple peaks in the epidemic curve" is incorrect. Point source epidemics typically have a single peak, not multiple peaks.
* **Option B:** "Produce multiple peaks in the epidemic curve" is incorrect, as point source epidemics typically have a single peak.
* **Option C:** "Be explosive" is correct, as point source epidemics can be explosive in nature due to the simultaneous exposure of a group of people.
* **Option D:** "Tail gradually" is incorrect, as point source epidemics typically have a sharp decline in cases once the incubation period has passed.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that point source epidemics are often caused by contaminated food or water, and they can be prevented by identifying and isolating the source of the contamination.

**✓ Correct Answer: B. ac**