如图，△ABC，△EFG均是边长为2的等边三角形，点D是边BC、EF的中点，直线AG、FC相交于点M．当△EFG绕点D旋转时，线段BM长的最小值是（　　）

阅读短文，从每小题所给的四个选项中，选出能回答所提问题的最佳答案。

    There are fifty students in our class. There are twenty-four boys and twenty-six girls. Some students live near the school, and some others live far from school. About half of the students usually come to school by bike. They often get to school at a quarter to seven. About fifteen students often come to school by bus. They often get to school very early, too. Another ten students come to school on foot. Their homes are near the school, but they are often late for school, because they get up late.

三大改造指的是（   ）