Beginning in 1918, the relevant personnel measured the Leaning Tower of Pisa every year. During the 40 years from 1918 to 1958, the average tilt was 1.1 mm per year; and for the 10 years from 1959 to 1969, the average tilt was 1.26 mm per year. Now the top center is perpendicular to the center. The line slopes 4.5 meters south. What does the above text mean? ['The south tilt of the Leaning Tower of Pisa is already very serious.', 'Increasing speed of leaning tower of Pisa.', 'Increasing accuracy of Leaning Tower of Pisa measurements.', 'Increasing efforts to measure Leaning Tower of Pisa.']