题目内容：
根据以下内容，回答题。Passage Four
Man's first real invention, and one of the most important inventions in history, was the wheel. All transportation and every machine in the world depend on it. The wheel is the simplest yet perhaps the most remarkable of all inventions, because there are no wheels in nature-no living thing was ever created with wheels. How, then, did man come to invent the wheel? Perhaps some early hunters found that they could roll the carcass of a heavy animal through the forest on logs more easily than they could carry it. However, the logs themselves weighed a lot.
It must have taken a great prehistoric thinker to imagine two thin slices of log connected, at their centers by a string stick. This would roll along just as the logs did, yet be much lighter and easier to handle. Thus the wheel and axle came into being and with them the first carts.
The wheel is important because__________. A．it was man's first real invention
B．all transportation depends on it
C．every machine depends on it
D．both B and C
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