问题描述：
thanks to the internet different kinds of information --- in a short time
a\ can be learned
b\ has been learned
c\ can learn
d\ has leasrned
请您说明理由好吗?
a\ can be learned
b\ has been learned
c\ can learn
d\ has leasrned
请您说明理由好吗?
问题解答：
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