Dan and Kim are friends. Dan lives in an apartment downtown(市中心). It is very convenient to go to work or go shopping, but it's too noisy. There is a lot of traffic. Sometimes Dan can't stand(忍受). Kim has a house in the suburbs(郊区). It's not convenient to go to work or go shopping, but it's very quiet. There is not much to do. In fact, nothing ever happens.

    Dan and Kim have a good idea. They decide to trade(决定交换)places one week.

植物体的整个生命历程中，植物激素起到了关键作用，以下说法正确的是（　　）

① $\overrightarrow{a}$ 与 $\overrightarrow{b}$ 的夹角； 

②求| $\overrightarrow{a}$ + $\overrightarrow{b}$ |和| $\overrightarrow{a}$ ﹣ $\overrightarrow{b}$ |．