题目内容：
根据以下内容，回答题。Passage Three
The first true piece of sports equipment that man invented was the ball.
In ancient Egypt, as everywhere, pitching stones was a favorite children's game. But a badly thrown rock could hurt a child. Looking for something less dangerous to throw, the Egyptians made what were probably the first balls.
At first, balls were made of grass or leaves held together by vines. Later they were made of piece of animal skin sewed together and stuffed with feathers or hay.
Even though the Egyptians were warlike, they found time for peaceful games. Before long they had developed a number of ball games, each with its own set of rules. Perhaps they played ball more for instruction than for fun. Ball playing was thought of mainly as a way to teach young men the speed and skill they would need for war.
The ball was probably invented because__________. A．throwing stones often caused injuries
B．throwing stones was not fun
C．games with stones did not have rules
D．rocks were too heavy to throw
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