题目内容：

    At some point in elementary school, your science teacher may explained to you that there are 365 days in a year because that's how long it takes for Earth to complete one full journey around the sun. What they might not have explained, however, is that it's not exactly 365 days—it’s actually closer to 365. 2421 days.

So, if we want our calendar year to begin right when Earth begins a new turn around the sun, we have to account for roughly an extra quarter of a day each year, or one day every four years. History.com reports that the Egyptians had already been doing this for a while before Europe finally caught on in 46 BC, when Roman Emperor Julius Caesar and astronomer Sosigenes put their heads together to come up with what we now call the Julian calendar, which includes 12 months, 365 days, and an additional ''leap day'' every four years on February 29.

But rounding 0.2421 up to 0.25 each year created an issue, because it didn't quite add up to a full day every four years—and that tiny difference meant that after 128 years, the calendar year ended up starting a day before Earth had completed its journey around the sun. By the 14th century, the calendar year was starting 10 days before Earth finished its orbit.

In 1582, Pope Gregory XIII sought to correct the error by suggesting that we simply skip a leap day every so often. His Gregorian calendar, which we still use today, rules that we skip the leap day during years which can be evenly divided (被整除) by 100 but not by 400. For instance, the year 2000 included a leap day because it can be divided by 100 and 400; the year 2100, on the other hand, will not include a leap day, since it's evenly divided by 100, but not by 400.

Gregory XIII’s correction to Caesar's overcorrection is itself a bit of an under-correction, so we'll probably need to reevaluate our leap day design again in about 10,000 years.

1.What do the underlined words ''put their heads together'' in paragraph 2 mean?

A.Come across each other. B.Bump their heads together.

C.Compete against each other. D.Combine their efforts.

2.According to Gregorian calendar, which year may include a leap day?

A.2100. B.1600.

C.1800. D.2020.

3.What can we learn from the passage?

A.Emperor Julius Caesar was also an astronomer.

B.Today's calendar exactly agrees with the sun's turning.

C.The 10-day early start in calendar will always exist.

D.The Gregorian calendar is not yet the most accurate.

4.What's the purpose of this passage?

A.To point out a mistake made by your science teacher.

B.To explain how long Earth makes a trip around the sun.

C.To explain in detail why the leap day is needed.

D.To explain the reason why we need a calendar.