PP test one--------68

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.

answer: B  steamship------------steamships

but why not A?  evolved-----------was evolved??

thanks!!

这里作“演变”讲，不及物，

下面摘自金山词霸：

evolve
vi.
演变, 逐渐发展
进化
The British political system has evolved over several centuries.
英国政治体制是经过几个世纪逐步形成的。

有不少单词既可以做及物动词，也可以做不及物动词，以后遇到类似问题，查一下字典就明白了