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My guess would be C) Thoroughly
Your Answer Passage:
First of all, you get a nice good bike. Then, you take the help of one or two people and try to ride around in a small area. Then, you take short times of balancing yourself on the bike by putting you center of mass at the wheels and not the floor. Then, you start to ride slowly. Last but not least, then after a lot and lot of practice like that, you start to jump of ramps and do flips. And fly at insane speeds.
I would go with D for the first one and B for the second one.
Because you gave not enough information to answer this question