A：Cu2+、Ba2+、OH-、Cl-
B：Na+、K+、H+、HCO-3
C：Mg2+、K+、Cl-、NO-3
D：H+、SiO2-3、NO-3、SO2-4
根据句意及首字母提示完成单词
①He asked p____ to leave the room.
② The government still face very s____ difficulties.
③We p____ £35 for each ticket.
④The country is facing great economic d____.
⑤Peter o____ to teach them water-skiing.
关于声音的产生与传播，下列说法正确的是（）
