Writing and Reading

      22In the northern hemisphere, the beginning of spring is marked by the passage of the vernal equinox, which is when the sun's path is directly over the earth's equator. It is one of two times during the year when day and night are more or less equal, and it is significant date on calendars around the world, falling between March 19 and 22 every year. The increasing daylight means the return of life, and those signs of life are how we know that spring has really arrived. There will be buds, flowers, bees, and of course, eggs. 

Which choice most effectively establishes the main topic of the paragraph?