国家开放大学 / 综合英语（1）
正确率：100%
The ball was probably invented because _ _ _ _ _ _ _ _ .
a.games with stones did not have rules
b.rocks were too heavy to throw
c.throwing stones often caused injuries
d.throwing stones was not fun
a.games with stones did not have rules
b.rocks were too heavy to throw
c.throwing stones often caused injuries
d.throwing stones was not fun
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