_______ porpoises and dolphins, whales are mammals.

The Romans used central heating systems very much like_______.

_______plants, which manufacture their own food, animals obtain nourishment by acquiring and digesting their food.

These two girls_______ that I can't tell them apart.

Nerve cells, or neurons, _______ in the human body.

The deep oceans contain some of the_______ of all living creatures.

Apart from Pluto, the outer planets_______ the inner planets and are made mainly of lighter materials such as

hydrogen and helium.

The younger you are, _______ it is to learn.

A baby might show fear of an unfamiliar adult, _______ he is likely t smile and reach our to another infant.

This is_______ man of all I've been known.

The United States spends more money on advertising_______ country in the world.

A microscope can reveal vastly_______ detail than is visible to the naked eye.

This house is_______ the others we've seen.

That child behaves_______ of all

It is_______ that I have read it twice.

The higher one rises in the atmosphere, _______ the temperature generally becomes.