1. My teacher had handed out the worksheets by the time l arrived at the class(QKHT)

Giải thích : S+ had + P2 + O

2.My brother's favourite reality show had finished before he turned the television on 

Giải thích : QKHT + Before + QKĐ( Trước before dùng quá khứ hoàn thành sau before dùng quá khứ đơn )

3.Before the students started with their exam,the teacher had collected their mobile phones and bags.

( Giải thích giống câu 2 ) 

1 A

Có by the time ->thì Quá khứ hoàn thành : S+had+P2+O

2 D

Theo công thức QKHT+before +QKĐ

3 C
Theo công thức QKHT+before+QKĐ