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A.these
B.不填
C.what
D.when
ace level (i.e. the complement of a mark) for one element period. This element is called the start bit and has a duration of T seconds. The transmitter then sends the character, l bit at a time, by placing each successive bit on the line for a duration of T seconds, until all bits have been transmitted. Then a single parity bit is calculated by the transmitter and sent after the data bits. Finally, the transmitter sends a stop bit at a mark level (i.e. the same level as the idle state) for one or two bit periods. Now the transmitter may send another character whenever it wishes.
At the receiving end of an asynchronous serial data link, the receiver continually monitors the line looking for a start bit. Once the start bit has been detected, the receiver waits until the end of the start bit and then samples the next N bits at their centers, using a clock generated locally by the receiver. As each incoming bit is sampled, it is used to construct a new character. When the received character has been assembled, its parity is calculated and compared with the received parity bit following the character. If they are not equal, a parity error flag is set to indicate a transmission error.
(1).When the transmitter wishes to send data, it first ().
A.places the line in a space level
B.puts the line in a mark level
C.sends a stop bit
D.places a bit at logical l level
(2).Then a single parity bit is calculated by the transmitter and ().
A.compared with the received parity bit
B.sent with the data bits
C.sent after the data bits
D.sent before the data bits
(3).At the receiving end of an asynchronous serial data link, the receiver continually monitors the line().
A.waiting for the end of the start bit
B.looking for a start bit
C.generating a clock
D.constructing a new character
(4).When the transmitter wishes to send data , it first sends().
A.a character at the same time
B.a character to the line
C.the start bit
D.a mark level to the line
(5).The transmitter then send the character, for a duration of T seconds().
A.each bit
B.all bits
C.successive bits
D.a start bit and a character
A study published recently in the journal Nature shows that air- borne leaded gas emissions from the United States were the leading contributor to the high concentration of lead in the snow in Greenland. The new study is a result of the continued research led by Dr. Charles Boutron, an expert on the impact of heavy metals on the environment at the National Center for Scientific Research in France. A study by Dr. Boutron published in 1991 showed that lead levels in arctic snow were declining.
In his new study, Dr. Boutron found the ratios of the different forms of lead in the leaded gasoline used in the United States were different from the ratios of European, Asian and Canadian gasoline and thus enabled scientists to differentiate the lead sources. The dominant lead ratio found in Greenland snow matched that found in gasoline from the United States.
In a study published in the. journal Ambio, scientists found that lead levels in soil in the North-eastern United States had decreased markedly since the introduction of unleaded gasoline.
Many scientists had believed that the lead would stay in soil and snow for a longer period.
The authors of the Ambio study examined samples of the upper layers of soil taken from the same sites of 30 forest floors in New England, New York and Pennsylvania in 1980 and in 1990.
The forest environment processed and redistributed the lead faster than the scientists had expected.
Scientists say both studies demonstrate that certain parts of the ecosystem respond rapidly to reductions in atmospheric pollution, but that these findings should not be used as a license to pollute.
1. Lead accumulation worldwide decreased significantly after the use of unleaded gas in the US .
A、 was discouraged
B、was enforced by law
C、was prohibited by law
D、 was introduced
2. It can be inferred from the last paragraph that scientists .
A、are puzzled by the mystery of forest pollution
B、feel relieved by the use of unleaded gasoline
C、still consider lead pollution a problem
D、lack sufficient means to combat lead pollution
