Once, they were made entirely of natural rubber.

以往，轮胎完全由天然制成。

Until recently, sneakers used natural rubber for this process.

很长时间以来，在这个过程中，运动鞋的制作都使用天然。

But today, most outer soles are made with a synthetic blend of natural rubber and byproducts from coal and oil.

如今，大多数外底材料由天然和煤及石油的副产品混合而成。

It is usually cheaper than natural rubber.

它通常比天然便宜。

Some of that will be natural rubber obtained from the sap of rubber trees.

其中一些将从树的汁液中提取的天然。

People get natural rubber from rubber trees as a white, milky liquid, which is called latex.

人们从树上获取天然，呈白色乳状液体，称为乳。

Today, the world needs so much rubber that we use both natural and artificial rubber in large amounts.

今天，世界需要如此多的，以至于我们大量使用天然和人造。