She had three sons, all   （）       became doctors.

题目解析 题目：She had three sons, all （） became doctors. 选项：A. who B. whose C. of which D. of whom 答案：D 解析：这也是一个定语从句的填空题。句子中有一个关系代词"all"，需要填入的是一个关系代词作为连接词，将"all"和"became doctors"连接起来形成定语从句。关系代词在这里充当宾格形式，指代前面的"three sons"，应该是"of whom"，意为"其中的"。其他选项不符合句意。