回答

[A]
1. By whom was the island discovered?
2. We can see the castle from here. ← The castle can be seen from here (by us).
3. The bear has been killed by the hunter. <現在完了・受動態>
4. I was spoken to by a stranger at the bus stop. ※speak to 人(人に話しかける):連語表現なのでこの形を維持する
[B]
1. It (must) (be) (done) at once.
2. (I'm) (interested) (in) the study of law. ※be interested in … …に興味がある
3. (It) (is) (said) that he is a great composer. ※They say that+S+V~=It is said that+S+V~ Sは~だと言われている

参考にしてください。

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