A:there were
B:has there bee
C:there has bee
D:were there
用元素符号或化学式填空.
(1)2个氢氧根离子____ ;(2)干冰____ ;
(3)硅元素____ ;(4)两个五氧化二磷分子 ____ .
I'm sure you can did it ____ (good).
小飞同学想研究小灯泡的发光亮度与实际电功率的关系,选取了一个标有“2.5V”、额定功率约为1W的小灯泡和必要的实验器材.