**Core Concept**
A cross-cylinder is a type of ophthalmic lens used in refraction to fine-tune the measurement of refractive error. It is a combination of two cylindrical lenses, one with positive and the other with negative power, used to neutralize or "balance" the cylinder axis.

**Why the Correct Answer is Right**
The most convenient form of a cross-cylinder is a combination of two lenses, one with a plus cylinder (positive power) and the other with a minus cylinder (negative power). This combination allows for a more efficient and accurate measurement of refractive error, as it enables the examiner to quickly determine the correct axis of the cylinder. The plus and minus cylinders are often oriented at 90 degrees to each other, which simplifies the process of balancing the cylinder axis.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not relevant to the concept of a cross-cylinder.
* **Option B:** This option may be a type of lens, but it is not the correct combination for a cross-cylinder.
* **Option C:** This option is also not relevant to the concept of a cross-cylinder.

**Clinical Pearl / High-Yield Fact**
When using a cross-cylinder, it's essential to remember that the plus and minus cylinders should be oriented at 90 degrees to each other. This allows for a more efficient and accurate measurement of refractive error and helps to prevent errors in axis determination.

**Correct Answer:** C.