Assertion-reason type statements should be evaluated independently for truth, and then separately for whether one logically explains the other. The diurnal (daily) variation of Earth's magnetic field is driven by solar-heating-induced ionospheric current systems, which is a distinct physical mechanism from the thermal demagnetization (Curie point) behaviour of magnetic minerals.
Statement I is true: the diurnal variation of Earth's magnetic field is indeed stronger during the day than at night. This is because solar (UV/X-ray)...
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