(1) The school (that/which) he once studied in is very famous.
(2) The school in which he once studied is very famous.
(3) Tomorrow I will bring here a magazine (that/which) you asked for.
(4) Tomorrow I will bring here a magazine for which you asked.
(5) We'll go to hear the famous singer (whom/that/who) we have often talked about.
(6) We'll go to hear the famous singer about whom we have often talked.
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