John looked unhappy when i went to see him yesterdayJohn looked unhappy when i went to see him yesterday,but I didn't know what he _______(think ) about.请回答尽可能详细,包括答案和解析,
问题描述：
John looked unhappy when i went to see him yesterday
John looked unhappy when i went to see him yesterday,but I didn't know what he _______(think ) about.
请回答尽可能详细,包括答案和解析,
答