The history of transportation is very long and full of changes and inventions. It starts _

__46__ walking, which is not any invention; it just takes energy. People used to walk to get to other places. If you wanted to get somewhere quickly, the __47___way to do that was to run . Actually, the first invention for the transportation __48__ was the shoe. Centuries ago there were no shoes, and people walked barefoot.

Then people invented ___49__ to transport themselves and materials from one place to another. In some cultures, people invented sledges(雪橇), ___50__ are a kind of board that you drag along the ground. You can tie things on the sledges to help carry them, but it’s a challenging invention ___51__ if you hit a rock with the sledges as you pull it, the contents can ___52___. In other cultures, people invented the wheels, which they used to make it easier to move things---and people. That was the beginning of many innovations in transportation.

___53___ people had wheels they could invent other ways to travel. They could put the wheels on a board and make it a wagon, and then they could ___54__ that wagon to an ox or a horse and ride as well as carry materials. That wheel led to __55___ we have today: trucks, automobiles, and even boats and planes. For example, there were steamboats that used giant wheels that turned with blades, pushing the water and pushing the boat forward.

46、_________

A.on

B.at

C.for

D.with

47A.possible

B.only

C.one

D.just

48A.probably

B.luckily

C.really

D.formerly

50A.what

B.when

C.where

D.which

49A.methods

B.channels

C.ways

D.measures

51A.unless

B.but

C.even

D.since

52A.pull out

B.slip off

C.hold down

D.put down

53A.Once

B.Then

C.However

D.Yet

54A.stick

B.make

C.fasten

D.change

55A.what

B.that

C.which

D.how

请帮忙给出每个问题的正确答案和分析，谢谢！