Why Was the Problem of Devising a Workable Yearly Calendar

Multiple Choice

Why was the problem of devising a workable yearly calendar so difficult on Earth?

A) the Earth's rotation period does not divide evenly into the Earth's period of revolution
B) The day is based on what the Sun does, but the year is based on what the Moon does
C) The length of the Earth's year varies from year to year
D) The calendar only works at Greenwich, England, and it gets further and further off as you move eastward in longitude
E) Various Popes of the Catholic Church disagreed about how long the day should be