The father was ill in hospital, all of whose sons looked after him . A. in turn

byturns意为“轮流着”，而in turn表示“依次，一个接一个”，相当于oneby one，故选D项。

Taketurns to do sth.  /at doing sth. 为动词短语，意为“轮流干某事”，相当于do sth. by turns。例如：

Onthe way home, they took turns at driving the car.

在回家的路上，他们轮流开车。

Thetwo children took turns to deliver letters for the old postman.

那两个小孩轮流着替那位老邮递员送信。

“Itis one’s turn to do sth. ”也是一个常用句式，意为“轮到某人做某事了”。例如：

It’syour turn to clean the blackboard.

轮到你擦黑板了。

Whoseturn is it to answer the question?

该轮到谁回答问题了？