问题描述：
He sang just now.(用when I came here 改写句子） He _______ ________ when I came here
He cleaned the blackboard yesterday.(用at this time yesterday代替yesterday)
He ______ _______ the blackboard at this time yesterday
He cleaned the blackboard yesterday.(用at this time yesterday代替yesterday)
He ______ _______ the blackboard at this time yesterday
问题解答：
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