He preferred to study music. He made it very clear in his letter

He preferred to study music. He made it very clear in his letter to his parents, (complex sentence)
【正确答案】：He made it very clear in his letter to his parents that he preferred to study music./He preferred to study music, which he made very clear in his letter to his parents.
【题目解析】：本句的要求是将两个单句变成一个复合句（complex sentence）。我们可以将其变成一个宾语从句He made it very clear in his letter to his parents that he preferred to study music.也可以将其变成一个非限定性定语从句He preferred to study music, which he made very clear in his letter to his parents.