Groundwater, a resource that exists everywhere beneath the Earth’s surface, is under increasing risk from contamination and overuse.

单项选择题The new theory was corroborated.

单项选择题A person who deals with the public must be courteous at all times, even when he or she is very tired.

单项选择题Cold air causes the arteries around the heart to constrict.

单项选择题The graduate students will convene in the Student Union.

单项选择题Some birds consistently return to the same nesting area each spring.