**Core Concept**
Cross cylinders are a type of ophthalmic instrument used in refraction to determine the correct cylinder power and axis for correcting astigmatism. They consist of two cylinders of equal strength but opposite sign (+ and -), which are used together to neutralize the astigmatic error.

**Why the Correct Answer is Right**
When using cross cylinders, the patient is presented with a combination of a plus and a minus cylinder of equal strength. By rotating the cylinders, the examiner can determine the correct axis of the astigmatism. The equal strengths of the cylinders allow for a symmetrical correction, effectively canceling out the astigmatic error. The combination of a plus and a minus cylinder also allows for a more precise determination of the correct cylinder power.

**Why Each Wrong Option is Incorrect**
**Option A:** This option incorrectly states that cross cylinders have unequal strengths, which would not allow for a symmetrical correction.
**Option B:** This option suggests that cross cylinders have unequal strengths, which is incorrect. Additionally, using unequal strengths would not allow for a precise determination of the correct cylinder power.
**Option C:** This option is incorrect because cross cylinders do not consist of two plus cylinders. The combination of a plus and a minus cylinder is essential for determining the correct cylinder power and axis.
**Option D:** This option is incorrect because cross cylinders do not consist of two minus cylinders. The combination of a plus and a minus cylinder is necessary for a symmetrical correction.

**Clinical Pearl / High-Yield Fact**
When using cross cylinders, it's essential to remember that the correct axis of the astigmatism is determined by the axis of the cylinder that, when added to the existing astigmatism, results in a symmetrical correction.

✓ Correct Answer: A. One plus cylinder and one minus cylinder of equal strength