Agronomists work to improve the quality of crops, increase the yield of fields, and ________ of the soil.

Nitric acid ________ copper to give off brown fumes of nitrogen dioxide.

Although the name was not popularized until the Middle Ages, engineering ________ civilization.

Penguins, the most highly specialized of all aquatic birds, may live ________ twenty years.

A desert receives less than twenty-five ________ of rainfall every year.

Most beekeepers have observed ________ at the approach of a thunderstorm.

The jaw structure of a snake permits it to eat and digest animals much larger than ________ .

When a woman becomes pregnant ________ in life, she encounters additional risks in delivering a healthy baby.

________ of Country-Western singers may be related to old English ballads.

The United States is ________ that there are five time zones.