**Core Concept**
The Fitzgerald and Hallpike differential caloric test is a method used to assess the vestibular function of the inner ear by stimulating the horizontal semicircular canal with warm or cold water. This test relies on the temperature-induced convection currents in the endolymph of the semicircular canal, which in turn cause the hair cells to bend, generating an electrical signal that is transmitted to the brain.

**Why the Correct Answer is Right**
When cold water at 30 degrees centigrade is irrigated into the left ear of a normal person, it will cause a nystagmus response in the left ear as the cold stimulus causes the endolymph in the left horizontal semicircular canal to become denser than the surrounding fluid, resulting in a convection current that bends the hair cells towards the utricle. This results in a beat-to-beat nystagmus response that is away from the irrigated ear (left) and towards the other ear (right).

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because the question is asking for the nystagmus response in a normal person, not in a patient with a vestibular disorder.
**Option B:** Incorrect because the cold-water irrigation would cause a nystagmus response in the left ear, not the right ear.
**Option C:** Incorrect because the temperature of the water is not the primary factor in determining the direction of the nystagmus response; rather, it is the direction of the temperature gradient that is critical.

**Clinical Pearl / High-Yield Fact**
It is essential to note that the direction of the nystagmus response in the Fitzgerald and Hallpike differential caloric test is determined by the direction of the temperature gradient, not the absolute temperature of the water. This is a critical distinction to make, as it can be easily confused with the direction of the nystagmus response.

**Correct Answer: B. Towards the opposite ear.**