Questions

Tether is a rope, chain, or the like, by which an animal is fastened to a fixed object so as to limit its range of movement.

Cord is rope like to tie. Hence, cord is most similar in meaning to tether.

When water freezes its density decreases.

Water at ordinary temperatures contracts and increases in density as it is cooled, like most substances. But at about 4°C it reaches a maximum density and then decreases in density as it approaches the freezing point.

Australia is the smallest of all the continents

The Ozone layer which is present in the Earth's atmosphere protects it by restricting from UV or Ultraviolet radiation, the dangerous rays contained by Sun.