81. Milk is indispensable _____ the health of a motherless baby.

A. to

B. at

C. with

D. on

82. Modern industry abounds _____ opportunities for young men with imagination.

A. for

B. on

C. in

D. of

83. More than one third of the Chinese in the United States live in California, ______ in San Francisco.

A. previously

B. predominantly

C. practically

D. permanently

84. More than two hundred years ago the United States _____ from the British Empire and became an independent country.

A. got off

B. pulled down

C. broke away

D. dropped off

85. Most good writers use every means _____ to make the reader’s way smooth and easy.

A. at their disposal

B. at their request

C. at their will

D. at their convenience