乾隆二十二年（1757）四月，英国商人洪仁辉第二次率领商船到宁波贸易。 乾隆皇帝担忧“洋船至宁波者甚多。将来番船云集，留住日久，将又成粤省一澳门矣，于海疆重地，民风土俗，均有关系”，于是，下令实行“一口通商”政策。这表明乾隆实行闭关政策的主要原因是(　　)

算式：____

补全对话

从方框中选择最佳选项完成对话，有两项多余

a. Yes!

b. The keys? They're on the table.

c. No, they aren't.

d. How about my books?

e. It's under the desk.

f. Where are they?

g. Thank you very much.

A: Hi, Susan! My keys（钥匙） are lost（丢失）. ____

B: Are they on the table?

A:____

B: En. . . Are they in your bag?

A: But where is my bag?

B:____

A:____They are here. But there aren't any books in my bag.

B: Ha- ha. Are they lost, too?

A: Oh, no.

B: Look! They are on the shelf (架子).

A:____

B: Not at all.

我国热带雨林保存最完整的地方位于（　　）