While I was walking on the street ,I met Mr Liu （改为同义句）

题目

While I was walking on the street ,I met Mr Liu （改为同义句）
I met Mr Liu ______（一词）I was walking in the street.
请不会的不要乱发表,我有正确答案,只不过想证实下,另外请解释下理由,否则当乱猜处理

答案

when ,
届时,在那时：
He came at six,when the sun was setting.
他六点钟来的,那时太阳正落山.