钱币被称为“国家名片”，人民币则是中国的名片。目前人民币一共发行了五套，一些风景被印上了它们的背面。读下图，回答下列各题。

根据短文理解，从A、B、C、D四个选项中选择最佳答案填空。

    Mike is an Englishman. He lives in a 1 building in the 2 London. There are eighteen floors in the building and he lives on the fifteenth floor. He 3 a lift (电梯) to go up and down. He works very hard. He 4 to work early. Every day he leaves his     5 and walks to the lift. He gets into the lift. It  6 him down to the first floor. He gets out of the lift. Then he 7 to a bus stop. The bus stop is in front of a station. It is about two hundred meters from his home. 8, he catches the No. 11 bus to work. But sometimes he goes by bike. He works in a factory about ten 9 from his home. His work 10 at half past eight. And finishes at a quarter to five. He gets back home at half past five.

如图,在Rt△AOB中,OA=OB=$3\sqrt{2}$ ,⊙O的半径为1,点P是AB边上的动点,过点P作⊙O的一条切线PQ（点Q为切点）,则切线PQ的最小值为____ ．