[求助]两道PP structure

yvonne16

test1-68. 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，不明白为什么

128. Small to medium-sized marsh birds similar in

    body shape by cranes, rails are found throughout

    the world, except in the polar regions.

这句完全看不懂！！答案是B

yvonne16

果然是对rail不认识。谢谢！

还有，为什么steamship要用复数？前面的riverboat不是单数么？是不是因为steamship前面没有冠词？