有三个集气瓶,分别充满空气、氮气、氧气,用下列方法能最简单加以鉴别的是( )
A:测密度
B:带火星的木条
C:燃着的木条
D:澄清石灰水
如图,在Rt△ABC中,∠ACB=90°,AB的垂直平分线DE交AC于E,交BC的延长线于F,若∠F=30°,BE=4,则AD的长是( )
The old town has narrow streets and small houses _______ are built close to each other.