In such cases two principal factors are at work.

Of these, nine countries reported air pollution as being the principal factor responsible for weakening forest health.

This waste heat is one of the principal factors that affect tube life.

The renewed development of the market in the 1980s can be traced to three principal factors.

It is one of the principal factors that set them apart from adult killers.

One of the principal factors is the reliance on coal as the main source of power.

The thesis of the first part is that environmental coincidences are the principal factor in human history.

Circulation is one of the principal factors used to set advertising rates.

Nervous systems are one of the principal factors that limit shrinking body size.

The talks also underscore a principal factor behind the mergers, the high cost of technology.