是through,across
after we walked THROUGH the forest and ACROSS a river,we stopped to take a rest为什么，逗号并列什么？原句是错的，不规范。walked through the forest 和walked across a river是两个不同的动作，只不过谓语动词walked是一样的，这时候，就必须是walked through the forest and across a river,用and连接这两个短语。什么时候才是并列两个短语呢？记住：1 walked through A, across B , out of C AND onto / into D2 walked through the forest AND swam acoss the river最後一个与它前面的那个之间要用and连接'''''最後一个与它前面的那个之间要用and连接''''''什么意思？A,B and C -----B和CA and B -----A和BA,B,C and D-----C和D 相同成分的单词或短语之间，最後两个之间要用and连接