X线管的真空度应保持在()
A.133.3×10Pa
B.133.3×10Pa
C.133.3×10Pa
D.133.3×10Pa
E.133.3×10Pa
参考答案:E
- Maria dances so well. Who taught her?
- Nobody. She learned by _____. [ ]
A. her
B. hers
C. she
D. herself
1439到1440年,德国人()采用铅为材料,铸造字模,利用金属字模进行印刷,这是最早的凸版印刷实验。著名的(),就是用谷登堡活字和印刷架排印的现存最早的印刷书籍。