It's nice of you to ______ so much time showing me around your school.
A. take
B. spend
C. cost
D. have
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Mr. Black loved his daughter Helen very much. Every day he spent all of his free time1 with her. One night, he had much work to do,2 couldn't play with his daughter. In order to keep her3, he found a map of the world in a magazine and cut it into pieces. When he 4, Helen came running to him and was ready to play. Mr. Black said he had 5 time to play fight now. He told the girl to put the pieces of paper back together to a map of the world, and then they could both play.
在电火花加工中,工件一般杰电源的()
A. 正极,成为正极性加工
B. 负极,成为负极性加工
C. 正极,成为负极性加工
D. 负极,成为正极性加工
根据电火花放电的基本要求,电火花加工必需采用()电源。
A. 直流电源
B. 交流电源
C. 脉冲电源
D. 无
在电火花线切割加工中,采用正极性接法的目的有()
A. 提高加工速度
B. 减少电极丝的损耗
C. 提高加工精度
D. 表面粗超度变好