题目

Sao Paulo in Brazil is a very big city, and it’s growing fast. Every year there are more cars and motorcycles on the roads. But not enough new rules have been built. The city’s traffic has become so terrible that sometimes there are traffic jams（堵塞）100 kilometres long! Traveling across the city can take hours.But in a helicopter（直升飞机）, it can take just a few minutes. There are now about 500 helicopters in the city, and they can be seen- and heard- flying at all times of the day and night. Sao Paulo has become famous for having so many helicopters.Tiago Gomez is a Sao Pa甲、乙两种体积相等的实心球，用不同的物质制成．当天平的右盘放上两个甲球，左盘放上三个乙球时，天平恰好处于平衡状态，那么甲、乙两球密度之比是（　　）A．2：3B．3：2C．6：1D．1：6