---What made Jack look so worried?---______ his expensive and favourite ten-speed bike．A.Being lostB.Having been lostC.LosingD.To be lost
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C解析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．”。