Mr. Brown lives on Green Street now. He wants to send a postcard(寄明信片) to his pen friend in Toronto, but he doesn't know where the post office is. “ where is the post office？” he asks his new neighbor Joe. “I want to send a postcard to Toronto.” “The post office is quite far from here，” answers Joe. “If you only want to post some letters(信件) or postcards, you needn't go to the post office. You can put them into a mailbox（信箱）down the street.”

我国地域辽阔，地形类型多样．山地、____　、平原、____、丘陵五种地形，在我国都有分布．

下列对传染病和免疫的叙述中正确的（   ）