利用空间电荷补偿变压器稳定管电流的原理是( )
A. 管电压升高时,提高管灯丝加热电压
B. 管电压升高时,保持X管灯丝加热电压不变
C. 管电压升高时,降低管灯丝加热电压
D. 管电压不时,提高管灯丝加热电压
E. 管电压改变时,保持X管灯丝加热电压不变
Facebook was created in ______ on Harvard campus.
A. 2005
B. 2006
C. 2004
D. 2007
Twitter allow users to send and read short_______character messages called tweets.
A. 140
B. 130
C. 114
D. 104
______has more than 500 million monthly active users.
A. Qzone
B. Youku
C. Wechat
D. Weiboo