活动星系核的质量至少应该是太阳质量的多少倍?()
A、10
B、1000
C、1000000
D、1000000000.0
参考答案:C
This bridge _______ three years ago. Now it's a nice view in the town. [ ]
A. build
B. built
C. is built
D. was built
常见数字中继信令有哪几种().
A、1号信令
B、7号信令
C、PRI信令
D、模拟中继