求助，PP 1---3 8

ross1261

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, 但是B steamship 错在哪，单复数吗？D没有错吗？我觉得transport应该用复数，不明白，谢谢。

真老九

transport这里做“运输”讲，passenger transport就是客运的意思，