下列溶液有时呈黄色,其中由于久置时被空气中氧气氧化而变色的是( )
A:浓硝酸
B:硫酸亚铁
C:高锰酸钾
D:工业盐酸
He had hoped to have an opportunity to travel abroad for long. But it was a(n) ______ that he missed it when it came last month.
数一数,比一比。
有____只 有____只 有____只
____, ____
____>____ ____<____