根据短文内容，选择正确答案。

    Most people in Britain live in a small family groups. More than a quarter of houses in Britain have only got one person in them. Some of these are old people but some are people of twenty to thirty who choose to live alone. 35％of houses have two people in them. and another 17％have three people. 15％have four people in them, and the other homes have five or more.

The families in Britain are small. It is unusual for parents to have more than two children. When children are about eighteen or nineteen, they leave their parents' home, and they often go to a different city. Sometimes they only visit their parents two or three times a year.

已知：如图，△ABC中，中线BD和中线CE相交于点O，求证：BO=2DO．

小明制作了洋葱鳞片叶表皮细胞、人的口腔上皮细胞临时装片，在显微镜下看到的情况如图，请据图回答：