单项选择题

下列4种设备中，属于计算机输入设备的是：
A、UPS
B、服务器
C、绘图仪
D、扫描仪

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Choose the right conjunctions according to the context of this paragraph.
The poverty levels were found to be reduced. the unemployment levels remained high. the illiteracy levels also. __no changes in income distribution were observed during the time of this study.
A、Though; And; But
B、Even though; In addition; however，
C、Even though; In addition，; However
D、Though; In addition，; However，
E、Even though; In addition，; However，

单项选择题

逆变电路中改变两组开关的切换频率，即可改变输出交流电的（）。
A、大小
B、频率