a quantity of的意思是什么

造句：

1.The number of these cars means that traffic in the city will get heavier and heavier.

这些汽车的数量代表着该市交通会越来越拥堵。

2.They count their food to keep their prey from being hunted to extinction.

它们测算食物的数量,使自己的猎物不会由于过度捕杀而灭绝。

3.On the National Day 70 years ago, we had to fly twice because there were not enough planes.

70年前的国庆,我们曾因为飞机数量不够要飞两遍。