长城以北农作物一般只能一年一熟，长城以南大部分地区则两年三熟或一年两熟。造成这种现象的主要原因是（    ）

口语交际.根据上下文提示，在下面对话的空白外填入适当的话语（话语可以是句子、短语或词），使对话完整.

(Sam and Ted are talking on the phone.)

Sam: Hello, this is Sam,____ 

Ted: Hello, Sam! It's me, Ted. This term is over. I'm going to see my grandparents.

Sam: Oh, really?____ 

Ted: They live in Shanghai.

Sam:____ 

Ted: I'm going there by train.

Sam: How long does it take?

Ted: About 8 hours.

Sam: Who will meet you at the station?

Ted: My uncle.

Sam:____ 

Ted: It's quite hot and rainy.

Sam: You'd better take an umbrella and some thin clothes.

Ted: Yes, I will. By the way,____ 

Sam: I've no idea. Maybe my parents will have a plan.

 Ted: OK, see you.

下图是两种气体发生反应的微观示意图，其中不同的球代表不同种原子。下列有关该反应的说法中，不正确的是（　）