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cleanclear

one grammer from pprivew:
  

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

   why the answer is B ???   steamship-> steamships ???

dapangqiu

stemship  是单数名词必须受限或者变成复数

yibadao

yeah