(2009年高考辽宁、宁夏理综卷)如图所示，一导体圆环位于纸面内，O为圆心．环内两个圆心角为90°的扇形区域内分别有匀强磁场，两磁场磁感应强度的大小相等，方向相反且均与纸面垂直．导体杆OM可绕O转动，M端通过滑动触点与圆环良好接触．在圆心和圆环间连有电阻R.杆OM以匀角速度ω逆时针转动，t＝0时恰好在图示位置．规定从a到b流经电阻R的电流方向为正，圆环和导体杆的电阻忽略不计，则杆从t＝0开始转动一周的过程中，电流随ωt变化的图象是( )
C.
旋转的导体棒在磁场中切割磁感线，产生电动势，由右手定则可判断，开始时电流方向由b向a，为负方向，后来磁场方向相反，电流方向为正方向，因为只有开始的周期和第3个周期内导体棒切割磁感线．
| Smallpox was the first widespread disease to be eliminated by human intervention. A highly contagious viral disease, it was endemic in Europe, causing the death of millions of people until the development of vaccination by Edward Jenner around 1800. In many non- European nations, it remained a dreaded, often fatal illness until very recently. In May 1966, the World Health Organization (WHO), an agency of the United Nations, was authorized to initiate a global campaign to eradicate smallpox. The goal was to eliminate the disease in one decade. At the time, the disease posed a serious threat to people in thirty nations. More than 700 physicians, nurses, scientists, and other personnel from WHO joined about 200,000 health workers in the infected nations to battle the disease. Because similar projects for malaria and yellow fever had failed, few believed that smallpox could actually be eradicated, but eleven years after the initial organization of the campaign, no cases were reported in the field. The strategy was not only to provide mass vaccinations but also to isolate patients with active smallpox in order to contain the spread of the disease and to break the chain of human transmission. Rewards for reporting smallpox assisted in motivating the public to aid health workers. One by one, each smallpox victim was sought out, removed from contact with others and treated. At the same time, the entire village where the victim had lived was vaccinated. By April 1978, WHO officials announced that they had isolated the last known case of the disease, but health workers continued to search for new cases for two additional years to be completely sure. In May 1980, a formal statement was made to the global community. Today smallpox is no longer a threat to humanity. Routine vaccinations have been stopped worldwide. |
The word "isolated" underlined in Paragraph 4 most probably means ______ .
A．restored
B．separated
C．attended
D．located