**Core Concept**
Balanced stone, also known as dental stone, is a type of dental casting stone used in dentistry for creating accurate and durable dental models. It is a calcium sulfate hemihydrate-based material that undergoes a complex setting reaction, which can be influenced by various factors.

**Why the Correct Answer is Right**
The correct answer, option B, refers to the addition of accelerators or retarders to dental stone, which allows for controlled setting times. This is crucial in dental applications, as it enables the dentist to work efficiently without compromising the accuracy or quality of the final model. The presence of accelerators or retarders helps to regulate the setting reaction, allowing for a more balanced and predictable outcome. This, in turn, ensures that the dental stone sets at a rate that is compatible with the clinical workflow.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because balanced stone does not undergo rapid expansion. In fact, the setting reaction of dental stone is carefully controlled to prevent excessive expansion, which could compromise the accuracy of the final model.

**Option C:** This option is incorrect because the amount of water of hydration in dental stone is not directly controlled by the type of stone used. While the water content can affect the setting reaction, it is not a defining characteristic of balanced stone.

**Option D:** This option is incorrect because the crystals in dental stone are not all of uniform size. The setting reaction of dental stone involves the growth of crystals, which can vary in size and shape depending on various factors, including the type of stone used and the presence of accelerators or retarders.

**Clinical Pearl / High-Yield Fact**
One important aspect of dental stone is its ability to be mixed and matched with different accelerators or retarders to achieve the desired setting time. This flexibility allows dentists to work efficiently and effectively, while also ensuring that the final model meets the required standards of accuracy and quality.

**✓ Correct Answer: B. In which, accelerators or retarders have been added according to need**