Big Ben is not the name of a man. It's the name of a big clock in London. This clock has four faces. So, no matter(无论)where you stand, you can read the time on the face of Big Ben. Each face is the size of a bus. The hands are about four metres long. If you go to London, you may want to visit the Houses of Parliament(英国国会大厦). In that place you will find Big Ben sits at the top of the clock tower in the Houses of Parliament. You will probably hear it and see it. The big clock makes such a loud noise. It goes every quarter of an hour. Big Ben has been the most famous symbol(标志)of  London

食品安全与人体健康密切相关．下列说法正确的是（　　）

若a²=9，$\sqrt[3]{b}$=﹣2，则a+b=（　　）

如图，△ABC的边AC与⊙O相交于C，D两点，且经过圆心O，边AB与⊙O相切，切点为B．如果∠A=34°，那么∠C等于（　　）

下列四组多边形中，能密铺地面的是（　　）

①正六边形与正三角形；②正十二边形与正三角形；③正八边形与正方形；④正三角形与正方形．