【評論主題】20. ______ entering the concert hall, I noticed that refreshments were being served.(A) On (B) About

【評論內容】On/Upon +ving… ,S +過★★★★&☆☆☆;...

【評論主題】20. ______ entering the concert hall, I noticed that refreshments were being served.(A) On (B) About

【評論內容】

On/Upon +ving… ,S +過去式動詞 

表示 一…就…

112年威廉英文課本 p.162