单选题 Prof. White, my respected tutor, frequently reminds me to ______ myself of every chance to improve my English.

单选题 Corporations and labor unions have ______ great benefits upon their employees and members as well as upon the general public.

单选题 The city is an important railroad ______ and industrial and convention center.

单选题 Crisis would be the right term to describe the ______ in many animal species.

单选题 These causes produced the great change in the country that modernized the ______ of higher education from the mid-1860's to the mid-1880's.

单选题 Researchers discovered that plants infected with a virus give off a gas that ______ disease resistance in neighboring plants.

单选题 An increasing proportion of our population, unable to live without advanced medical ______, will become progressively more reliant on expensive technology.

单选题 The movement of the moon conveniently provided the unit of month, which was ______ from one new moon to the next.