Singapore lies at 1°N latitude, between the Tropic of Cancer and Tropic of Capricorn. The sun is directly overhead twice annually (approximately once each when the sun crosses the zenith going north and south) due to the parallelism of Earth's inclined axis - the tilt direction stays fixed as Earth orbits, causing different latitudes to face the sun directly at different times. Options A, B, and C don't explain this phenomenon.