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已知实信号x()的最高频率为fm (),则对信号x(2t), x()*x(2t)(卷积), x()·x(2t)(相乘)进行抽样,要求不混叠的最小抽样频率分别为( )

A. 2fm,4fm,6fm
B. 4fm,2fm,6fm
C. fm,2fm,3fm
D. 2fm,6fm,4fm

中国最早用铜版雕线技法制作的钱币是：What was the earliest coin made in China with intaglio engraving techniques

A. 人民币RMB
B. 大清银行兑换币Qing dynasty Bank Exchange Notes
C. 金圆币Gold yuan notes
D. 法币Legal currency

凹版的意思是凹处积墨转印的方式Gravure means the method of transfering ink in the pit

“海特”技法是由海特发明的Hayter technique was invented by Hayter