Is this problem （ ）the teacher explained yesterday?A、that B、the one
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Is this problem （ ）the teacher explained yesterday?
可以调一下语序
This problem is----the teacher explained yesterday.这时就很明显是B了.既要连接句子又要做后面句子中explained的宾语.
======以下答案可供参考======
供参考答案1:
因为这个问题老师老师昨天已经见过了，是个已经提出的问题
所以要用定冠词the,特指这个问题。