题干
The book is now out of print, ______ it can easily be borrowed from libraries.
A：and
B：for
C：so
D：but
答案(点此获取答案解析）
D
Sn2＋(aq)＋Pb(s)，体系中c(Pb2＋)和c(Sn2＋)变化关系如下图所示。