资料题：Which of the following usually happens on April 1 in Scotland（　）。

Long ago men learned that the world of nature is built with mathematical exactness according to refined and powerful laws: every bee strokes its wings exactly 440 times a second to propel itself forty miles an hour; the eye of every fly has exactly 70,000 lenses; every spider's thread is composed of 300 single threads.The progress of civilization itself can be measured by its range of mathematics.The early Egyptians knew enough of geometry to build the pyramids and the Greeks used trigonometry to measure the circumference of the earth two thousand years before Columbus sailed.European civilization combined geometry and trigonometry with the decimal system of the Arabs to produce the maps with which they explored the known world.In a later century, mathematicians studied the pendulum of the newly invented clock and created the language of calculus to measure motion.The yardstick of measurement today must cover everything from units tinier than atoms to distances of millions of light years.Speed and accuracy are vital.Man can never send a rocket to Mars unless he knows the exact distance to the planet, 8ince the slightest error could mean missing it by 30,000 miles.In navigating amid the planets and stars, man must have the help of some kind of machine calculators for fast and absolutely accurate information.