Chuyển từ câu chủ động -> bị động
1.Who helped her?
-->By whom she was helped ?
2.Who bake this cake?
-->By whom this cake is baked?
3.Who has written this sentence on the wall?
-->By whom this sentence has been written on the wall ?
sao lại dùng whom mà ko phải who
giải thích đc ko ạ