题目

阅读下面短文，在空白处填入1个适当的单词或括号内单词的正确形式。Chinese astronauts successfully carried out the country's first spacewalk outside their space station—a (significance) milestone in its rapidly expanding space program.The spacewalk is only the second by Chinese astronauts since the Shenzhou 7 manned mission (任务)of 2008, which (conduct) outside a spacecraft.China launched three astronauts into space in June. On Sunday morning, two of them left the core module(核心舱) of the space station called Tiangong. During spacewalk, they tested new-g在“探究小车速度随时间变化规律”的实验中，下列说法正确的是 (　　)A. 电火花计时器正常工作时，其打点的周期取决于交流电压的高低B. 电火花计时器正常工作时，其打点的周期取决于纸带运动的快慢C. 打点计时器只能连续工作很短时间，打点之后要立即关闭电源D. 电火花计时器应接220 V交流电