单选题
The term "virus" is derived from the Latin word for
poison, or slime. It was originally applied to the noxious stench emanating from
swamps that was thought to cause a variety of diseases in the centuries before
microbes were discovered and specifically linked to illness. But it was not
until almost the end of the nineteenth century that a true virus was
{{U}}proven{{/U}} to be the cause of a disease. The {{U}}nature{{/U}}
of viruses made them impossible to detect for many years, even after bacteria
had been discovered and studied. Not only are viruses too small to be seen with
a light microscope, they also cannot be detected through their biological
activity, except as it occurs in conjunction with other organisms. In fact,
viruses show no traces of biological activity by themselves. Unlike bacteria,
they are not living agents in the strictest sense. Viruses are very simple
pieces of organic material composed only nucleic acid, either DNA or RNA,
enclosed in a coat of, protein made up of simple structural units. They are
parasites, requiring human, animal or plant cells to live. The virus replicates
by attaching to a cell and injecting its nucleic acid; once inside the cell, the
DNA or RNA that contains the virus' genetic information takes over the cell's
biological machinery, and the cell begins to manufacture viral protein rather
than its own.
单选题
Which of the following is the best title for the passage?
单选题
The word "proven" (Line 4, Para. 1) is closest in meaning to which of
the following?
A. Shown.
B. Imagined.
C. Feared.
D. Considered.
【正确答案】
A
【答案解析】[解析] 本题考查语义判断。第一段最后一句：“But it was not until almost the end of the nineteenth century that a true virus was proven to be the cause of a disease．(但是，几乎直到19世纪末，人们才证明病毒导致疾病。)”可知[A]与文中proven的意思相近；而[B]，[C]和[D]的意思与proven相差较远。
单选题
The word "nature" (Line 1, Para. 2) is closest in meaning to which of
the following?
A. Self-sufficiency.
B. Characteristics.
C. Shapes.
D. Speed.
【正确答案】
B
【答案解析】[解析] 本题考查语义推断。第二段第一句：“The nature of viruses made them impossible to detect for many years，even after bacteria had been discovered and studied．(即使细菌被发现并被研究之后，病毒的特性也使它们很多年来不可能被察觉)。”由此可知nature意思为“特性”， [B]characteristics与之相近。
单选题
The author implies that bacteria were investigated earlier than viruses
because