With age, the mineral content of human bones decreases, ......... them more fragile.

.......................... many copper mines in the state of Arizona, a fact which contributes significantly to the state's economy.

.................... the Canadian composer Barbara Pentland wrote four symphonies, three concertos, and an opera, among other works.

The Chisos Mountains in Big Bend National park in Texas were created by volcanic eruptions that occurred .........................

Mango trees, ...................; densely covered with glossy leaves and bear small fragrant flowers, grow rapidly and can attain heights of up to 90 feet.

According to some indicators, the goal of teaching is to help students learn what ............. to know to live a well-adjusted and successful life.

the sapphire's transparency to ultraviolet and infrared radiation makes ...... in optical instruments.

........ initial recognition while still quite young.

Computers that once took up entire rooms are now .............. to put on desktops and into wristwatches.

The ponderosa pine is ...... of the most of the timber used by forest-product firms in the Black Hills of South Dakota.