单项选择题

IEEE 802.11g标准的物理层采用了扩频技术，工作在（）频段。

A．600MHz
B．800MHz
C．2.4GHz
D．19.2GHz

<上一题 目录 下一题>
热门 试题

单项选择题
Archimedes was a famous Greek mathematician and scientist. He was born around 287 BC and he died in the year 212 BC. Archimedes is most well-known for one specific idea that he came up with. 'Archimede's Principle' states that a solid object which is immersed in a liquid is pushed up by a force which is equal to the weight of the water that the object moves. For example, if you put a piece of wood and a piece of gold the same size in water, only the wood will float. Both the wood and gold move the same amount of water, but the wood weighs less than this water, while the gold weighs more. It is believed that Archimedes discovered this principle when the king of Syracuse asked him to solve a problem. The king wanted to know if his crown was pure gold or a mixture of gold and silver. The king, of course, did not melt his crown to find out. The idea came to Archimedes as he lowered himself into his bath. He noticed how the water spilled out of the tub. He decided to use the same idea for the crown. He knew that a gold crown immersed in water would weigh more than one made of silver. The experiment was done and the goldsmith was proved guilty of trying to cheat the king.A good title for the selection is ______.A．ArchimedesB．Archimedes' PrincipleC．A gold and silver crownD．The king of Syracuse
A.
B.
C.
A
D.
A．Archimedes
B．Archimedes'
E.A
F.The
单项选择题
From the beginning rivers have played an important part in the life of man. Man of the earliest times used the rivers as a means of travel. Today rivers still serve as a great waterway for the transport and people. In ancient times, man settled near rivers or on fiver banks and built up large empires. Water is Nature’s most precious gift to man. Man needs water to irrigate his crops, to cook and to wash. In nations all over the world rivers mean life and wealth. They feed and clothe the nations around them. Water is also a source of energy and power. Man constructs huge dams across the river to control the water for irrigation and get the energy needed to drive generators. The electrical power is then directed to homes, cities, factories and television stations. Man uses water each day. His main source of water comes from reservoirs, which in turn get their water from the rivers. Rivers also bring down soil and minerals from the mountains and deposit them on the plains building up rich fiver deltas for raising plants and crops. Fresh water life in rivers or in lakes fed by them provide man with food. In a small way rivers help to keep man in good health and provide for his amusements. Various forms of water sports keep man strong and healthy. Rivers have run on this earth long before man. Man’s future ability to live is uncertain, but rivers will flow on forever.Rivers have been important to man ______.A．since the last centuryB．for a very long timeC．since a few hundred years agoD．since a few years ago
A.
B.
C.
D.
E.
F.
G.
H.
Rivers
I.
A．since
J.for
K.since
L.since
相关试题