-What made Jack look so worried? - his expensive and favourite ten-speed bike． [

考查动名词作主语及省略。根据疑问词what可知，填上的部分需在句子中作主语，代替what，而having been lost强调动作先于主句的动作发生，重在两个动作的对比。答语实际上是“Losing his expensive and favourite ten-speed bike made Jack look so worried．”。