改错题请教，PP-1-68

The Mississippi riverboat, which (evolved) from simpler (steamship) of the early 1800's, became the (dominant) form of (passenger transport) on the Mississippi River.

答案是B,俺不明白，请大家指点迷津。

Thanks a lot.  

这个题大家讨论过，B是指名词单独出现的时候，单复数的要求。所以用streamships比较好。

请看PP语法笔记：http://forum.chasedream.com/dispbbs.asp?boardID=58&ID=48503&page=1