BY 1820, THERE WERE OVER SIXTY STEAMBOATS ON THE MISSISSIPPI RIVER...

49. - 2000 CAU TRAC NGHIEM ANH A B C
2000 CAU TRAC NGHIEM ANH A B C

49. By 1820, there were over sixty steamboats on the Mississippi river,

……… were quite luxurious.

a. many of them

b. which many

c. many of which

d. many that

 c