I was doing some shopping ____ I saw my old friends yesterday afternoon (when/while/as)

问题描述:

I was doing some shopping ____ I saw my old friends yesterday afternoon (when/while/as)

when
be doing.when 正在做.这时候突然.为什么不是while这是固定句型,在这里when的意思是“这时候突然。。。。”类似的句型还有be about to do......when ..=be on the point of doing sth......when...将要做。。。。这时候突然 。。。。had done sth...... when....刚做完.....这时候突然。。。。