根据下面内容，回答题：

Twelve years ago， oceanographer Captain Charles Moore was skippering his yacht the Alguita in the North Pacific. He sailed into a mass of floating plastic rubbish which took him and his crew a week to cross. This floating rubbish dump is now called the Great Pacific Garbage Patch and doubles the size of the USA.

The United Nations says there are now 18，000 pieces of plastic in every square kilometre of sea everywhere in the world. A walk along any beach will give you some idea of the seriousness of plastic pollution.

The trouble is， when we throw out plastic with the trash， the plastic doesn"t go away. Plastic does not biodegrade. It photo degrades into smaller and smaller particles which then enter the food chain.

Plastics contain cancer-causing chemicals along the food chain in increasing concentrations and end up in our fish and chips， along with hormone disruptors. Scientists try to tell us that we are killing ourselves as well as other animals. At least 200 species are， as I speak， being killed by plastic.

Whales， dolphins， turtles and albatross confuse floating plastic， especially shopping bags and six pack rings， with jellyfish.

Some countries have rebelled and banned plastic bags. And the first was brave Bangladesh. Then China took the same decision. Botswana， Canada， Israel， Kenya， Rwanda， Singapore and South Africa have also banned plastic bags. Notice how many of the world"s richest countries are not on this list. It"s an absolute disgrace.

Think globally， act locally. A small Australian town is now one step ahead of the rest of the world. The inhabitants of Bundanoon in New South Wales have banned plastic bottles from the town.

We need to follow their example and eliminate plastic from our lives， take care of the earth and vote for people we think will do the same.

The writer says that we can get an idea of how much plastic rubbish there is in the oceans by________. 查看材料