单选题When he was working 

问题补充：

单选题Whenhewasworkingtherehecaughtaseriousillnessfrom_____effortshestillsuffers.A.whichB.thatC.whoseD.what

答案：

C解析C。from whose efforts he still suffers 为修饰 a serious illness 的定语从句，whose 在定语从句中用作定语，修饰 efforts。