SAT语法题.2010年一月的.
In 1988 a Soviet icebreaking ship helped free three gray whales that had become trapped in the Arctic ice after they (had swam) into the coastal waters of Alaska to feed.had swam错了.我知道.可是我说不出原因 也不知道怎么改after后面只能接ING吗.还是就是单纯的时态不对?