A:Can you see my cat?
B:Did you see my cat?
C:Where is it?
两个氢原子____; 氖气____;两个铵根离子____;氧化镁中镁元素的化合价____;氯化钠中的阳离子____.
—Before he went on the stage,he took a deep ______ .
—He was too nervous.
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