题干
. They were surprised that a child could work out the problem ______ they themselves couldn’t.
A：once
B：while
C：the
D：if
答案(点此获取答案解析）
B
A：once
B：while
C：the
D：if
Na2S4+3NaBr。当闭合开关K时，X电极附近溶液变红。下列说法正确的是（ ）