One evening , my dad asked me to buy some bread for dinner . It was dark and cold outside . I rode a bike to a shop near my school . When I left the shop ,it got even darker , so I got on my bike right away. Then I found a woman in a white dress riding a motorcycle after me . She followed me for a long time . I rode very fast and started to cry for help, but no one was there . I was too scared and too tired to ride any faster . At last , I gave up . The woman stopped in front of me and said , “ Why were you riding so fast , Ken ?It’s dangerous !” I looked at the woman . “ Oh , It's you, mum !You really scared me . Dad said you wouldn't be back for dinner tonight !”

如图为生产实践中的几种育种方法的示意图，请据图分析回答下面的问题．

人民币上学地理，依人民币上的景观完成下面小题．

生活中发生的下列变化，属于化学变化的是

某人从A点出发向西走了10m，到达B点，然后改变方向按西偏北60°走了15m到达C点，最后又向东走了10米到达D点．

（1）作出向量$\overrightarrow{AB}$，$\overrightarrow{BC}$，$\overrightarrow{CD}$（用1cm长的线段代表10m长）

（2）求|$\overrightarrow{DA}$|．