51-55/11. . .

`51. C`. was going `-` lost

`52. A`. met `-` was working

`53. B`. were walking `-` asked

`54`. `C`. were `-` going `-` saw

`55. D`. rang `-` was working

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`***`

When/While QKTD, QKĐ

When/While QKĐ, QKTD

`->` QKĐ: S + V-ed/c2

`->` QKTD: S + was/were + V-ing

`color{red}{#lgdr}`

`51` `C` .

`52` `A` .

`53` `B` ,

`54` `C` .

`55` `D` .

`-` When/While + S + was/were + V_ing + O , S + V(quá khứ) + O : Khi ai đang làm gì , ai đó đã làm gì .

`-` While + S + was/were + V_ing + O , S + was/were + V_ing + O : Khi ai đó đang làm gì , ai đó cũng đang làm gì .