[求助]两道PP structure

   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

68. steamship --> steamships

128. by --> to. similar to 与……类似，固定搭配。Rail (一种鸟)，比中等大小的沼泽里的鸟类（类似鹤的身体那么大）要小，遍及世界各地，除了两极地区没有。
呵呵，翻译的乱七八糟的，但意思对了。^&^

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？